After upgrade to Ubuntu 11.10., your Avidemux is deleted.
To bring it back, go to following link https://launchpad.net/ubuntu/+source/avidemux/1:2.5.4-0ubuntu6/+build/2158020 and download
libavidemux0_2.5.4-0ubuntu6_i386.deb avidemux-common_2.5.4-0ubuntu6_all.deb avidemux-plugins-common_2.5.4-0ubuntu6_i386.deb avidemux-common_2.5.4-0ubuntu6_all.deb
and install these packages. This will bring Avidemux back.
Note: Make sure you still have libx264-106 package (from 11.04) installed. If you have newly installed Ubuntu 11.10, you have to get all required dependencies (open deb files and see INFO) that you are missing. Upgraded PCs usually have them from 11.04.