Ok, first of all to NU's problem, you cannot connect to any servers with Mountain Lion and there is no way around that for you I am afraid. Apparently the problem is quite stupid, the filename for the ET main menu is too long so you get an overflow when you read it and this will stop you from playing. It only appears for 10.8 because they changed the way how to read filenames or so.
About the problem with Lion Mac OS. I remember that it took me a while to find everything, but it is possible, because I play on 10.7.5 myself.
I downloaded ET from this location: http://returntocastl...itory_Mac;44985
The thing is that you have one folder in Applications and one folder is in Application Support which is located in ~/Library/Application Support
In order to find that folder, go to your Finder, go to the Menu Bar and press on Go and when you press Alt, Library will appear (it's hidden for some reason).
Anyway, in that Application Support you should find an Wolfenstein ET folder and a main folder, you need to copy your etkey into that folder, not in the Application folder.
After that, start the game and open console (with ~ key) and type the following:
/pb_cl_enable; /pb_system 1; /pb_writecfg; /reconnect
You should be able to see your guid with the following command:
You should be able to connect to Jaymod and NQ servers. In order to connect to silent servers, you will need to download the silent mod manually and copy the silent folder into your ~/Library/Application Support/Wolfenstein ET folder.
Let me know if you have any problems with that. You can also add me on xfire if you need help.