OK, go to 'install and remove software' > Options > Media manager, then configure your sources (mirror list). I would suggest you enable these:
Main
Main Updates
Contrib
Contrib Updates
Non-free
Non-free Updates
PLF Free
PLF Non-free
and disable anything else. Then in the same Media manager window, go to File > Update, and select what you want to update. I would suggest you select and update all the repos I've listed above.
Now try to install amule. Please let us know if that works?