Hi, you would just go to the program directory where the Bitcoin executable file is installed, and type
./bitcoin-qt -salvagewallet

or if it is in the path, you can just type from any directory in a terminal window:
bitcoin-qt -salvagewallet

As for going to the data directory to make a copy of the wallet.dat, you can do that in finder:

That folder is now invisible in Lion.  You can go there by entering a path in Go -> Go To Folder or by holding down option while the Go menu is open and then choose Library (which will disappear from that menu if you let go of option).

optionally, you can unhide the folder:
If you need to access the ~/Library folder from within an application, you can make the folder permanently visible. Just open Terminal and type in:

chflags nohidden /Users/[username]/Library/

Replace [username] with your user name(!) and hit return. Then you can quit Terminal and your user Library folder will be visible.


BTW, I do not use a mac, but I know how to use Google.