Here's what I did to run the jar I gave you under Linux:
Grab the latest swt build from:http://www.eclipse.org/downloads/download.php?file=/eclipse/downloads/drops/R-3.1.1-200509290840/swt-3.1.1-gtk-linux-x86.zip
Unzip it into a directory called "swt" in the current path.
java -cp swt:crosspost-2006011001.jar -Djava.library.path=swt com.abditory.crosspost.CrosspostMain
You should get an error about it not being able to find the text file for the license because you're not executing the jar directly. It's GPL, so only worry about it if you've never read through the license.
Problems like this are a pain because everybody's Java environment seems to be set up a little differently. I'm working on some docs (behind the scenes) that will go up on http://crosspost.sourceforge.net
to help out with issues like this. I also plan on doing some new packages that shoudl solve some of these problems. For example, there will eventually be a linux package that will provide a startup script for running Crosspost (much like Azureus, another SWT-based Java project, does).
Hope this helps. Keep me posted.