I do not have a Mac (used ResidualVM under Linux), so I cannot try it out, but the following should work:
1. Download ResidualVM
2. Install it shomehwere
(I have no idea how this works on a Mac, I think you simply open the dmsg file?)
3. Copy the Grim Fandango CDs on disk
in in principle you can simply copy all files into one directory. If your CD is copy-protected you might have to use some tools
4. Drop the patch into the same directory (no need to install it)
5. Run ResidualVM, point it to the directory you picked, enjoy the game.
I hope this works for you, I did this successfully under Linux a few month ago.