Accessing MacOS X clipboard from

Accessing MacOS X clipboard from

I'd like to access the system clipboard in Mountain Lion as a register in
vim.
The vim that comes with Mountain Lion is built with -xterm_clipboard shown
in vim --version. Homebrew has a formula for vim in the dupes collection
that also builds with -xterm_clipboard. After editing the formula I'm now
building vim with +xterm_clipboard. However, going into vim and using :reg
to see the registers doesn't show the contents of the system clipboard in
any register.
How can I get this to work?