Hi David,
"Meta" is the "Option"/"Alt" key on the Mac. The modifier "D" stands
for the "Command" key. So, no need to translate anything, just press
the key next to Cmd instead.
By the way, if you intend to use the GUI then you should know that the
Carbon version is horridly outdated. Go to
http://code.google.com/p/macvim/
for an up-to-date GUI version of Vim on the Mac. (If you do download
MacVim.app you might need to check out the 'macmeta' option to get
<M-..> mappings to work [requires the snapshot, not in the stable
build]. If you have any problems with MacVim you can ask on vim_mac.)
Björn