I had been trying to install the Jabber server and x11vnc but when I ran apt-get on either of these it could not be found. By default Xubuntu did not include the Universe repositories. Which are repositories of software that are not Officially supported by Ubuntu.
To include them find your Synaptic Package Manager and click on Settings and select Repositories. Scroll the list and put a check mark on the ones that are labeled Universe and Multiuniverse.
After I did this I was able to find the packages i was looking for.
