Hmmm...I don't think that I am necessarily the best one to explain how to use 'xterm' and shell commands for the purpose of helping you with mephemeris, but the basics are:

1) Install 'xterm' application (using gronmayer repository search site to look for it?)
2) Run 'xterm' application
3) Type the command '/usr/bin/mephemeris'
4) Write down and report the messages that occur when the application crashes

For myself, I use 'ssh' to connect wirelessly to my N800 for stuff like this -- since it gives me a nice big screen (my desktop monitor) rather than trying to use the tiny screen on the N800 -- but that is something that you might not be ready for yet