I discovered andLinux yesterday, got it installed, and found and fixed a ton of bugs which were keeping the compressor from working properly in linux, and making the GUI in linux look weird or screwed up. So, there's
a new version now, 3.3.2, which fixes those things. (I don't know why nobody reported those problems)
I also added improved tips for how to install Mono 1.2.6 on Ubuntu near the top of the page. I'll repaste it here:
If you use Ubuntu or the like, and 1.2.6 isn't in your repositories, go to the Mono website and click 'other downloads', and get the .bin file. Mark it executable, and then run it. It should install mono 1.2.6 for you. (For me, after it finished it said something about missing some libraries, but mono still ran the compressor fine - some other things, like monodevelop, didn't work though)
(Obviously, you don't need monodevelop to run the compressor)
Mono's downloads page is here: http://www.go-mono.com/mono-downloads/download.html
Mono also has a Mac OS X installer (if that's what a dmg file is) on their website - I haven't tried the compressor with mono on OS X (I don't have a mac, it isn't free, and I don't have an emulator for it or anything like that), but hopefully it should work just the same as it does on linux.