As far as we know, in many desktop manager an audio manager is provided (ex. aRts on KDE or ESD in GNOME). The audio manager often locks the audio resource for it's own use (this may happen while and after an application used it for playback). So, to solve the problem try disabling the audio manager and set all your audio application to use ALSA directly: ALSA is capable of sharing the audio resource out-of-the-box.

You can setup the desktop integration even for Single User Renoise installations with few steps.
Open a terminal and go into the directory where you have extracted the Renoise archive and type the following: Now you have to create a new desktop entry for Renoise. Start using the provided desktop entry as base; write the following: Now the boring stuff: editing the content of the desktop entry. Open the file renoise_single.desktop with your preferred editor. You have to modify two lines. Change the line that says: to and change the line that says: to Save and close the editor.
Let's now install the entries in desktop environment. Back into the terminal and type the following: Now your local installation of Renoise is integrated in your user desktop environment.

Renoise never needs a window or desktop environment to run. This gives you the possibility to run Renoise in a stand-alone environment. The primary advantage is that quite all the system resource are dedicated to Renoise (is that enough?).
If you are using a graphical login manager, a smart way to get a stand-alone Renoise environment is setting up a dedicated Renoise session. Prerequisites: you need a Renoise installed system-wide and you should be able to get root privileges.
Let's start. Open you favorite editor an write the following text in it: For example if Renoise is installed in /usr/local/bin the text above looks like this: Save this file naming it renoise.desktop. Open a terminal and get root privileges. Copy renoise.desktop file into the xsessions directory (it is usually located in /usr/share/xsessions/).
Now, when you log into your machine you will be able to change the session to "Renoise" and run it stand-alone.

To enable the Renoise ALSA realtime priority you need to run Renoise as root. This can be done by opening a terminal, becoming root (using su or sudo) and run Renoise. This method creates two copies of your configuration settings in the system (one copy for your user and one copy for root): this is ugly.
A nicer method is to set the Renoise executable SUID: this way when you run Renoise from your user environment it has root privileges. To make Renoise SUID do the following:
first, become root On Ubuntu you can get the root shell this way: second, verify Renoise executable is owned by root: check the 3rd and 4th column; they should be set to 'root', if not, and you want to change the ownership of the Renoise executable, do the following (PLEASE NOTE: if you have to do the following command because Renoise owner is not root there's something strange happening... if you want to continue do that at your own risk): third, make Renoise executable SUID: Now you can enable the ALSA realtime priority in Renoise using your own copy of your configuration settings.

Renoise never lists LADSPA plugins that have zero input and zero outputs. Also non-realtime capable plugins aren't listed as well. If your plugin has audio inputs and audio outputs, and it's realtime capable, it cannot be listed because it let Renoise crash.

While connecting to a LADSPA plugin Renoise can't detect which audio ports are REAL audio (left-/right-) inputs and which are REAL audio (left-/right-) outputs. So Renoise assumes the first two input audio ports are left-input and right-input and the first two output ports are left-output and right-output.
LADSPA plugins are designed to be something like basic blocks for synthesizer modelling environments. That means a single plugin can do just only logic operations (ring modulation between two audio signal, produce an ADSR envelope when an input audio signal is over defined threshold, and so on) or the plugin expects more than two audio in to produce its output (think about surround processors). That's why Renoise can list a LADSPA plugin which output has no sense at all.

The answer to this question is similar to the above ("Why Renoise uses the wrong in-/out- ports of my plugin?"): Renoise can't list generators as instrument because it can't detect what's the use of the plugin input control ports. Guessing the correct control port for setting the frequency by it's label isn't a solution: due to the nature of LADSPA plugs a frequency input can be control data as well as audio data. Also Renoise can't know which kind of control data the plugin expects: can be a signal that raise the freq by 3dB/octave, the MIDI note number or even pure frequency as a floating point number.
This is a part of the problem with LADSPA generators: another one is that note-on and note-off events are unsupported (in better cases you can have a control handling them) because LADSPA never describes instruments (unlike VSTi or DSSI).

Renoise features an active community where you can ask you questions. So don't hesitate joining the community: here's the starting point (click Community → Forums)