I have the GA-P55M-UD2 board, and referencing this photo I took of the BIOS I'm pretty sure it is F11. OS X 10.6.6.
I downloaded DSDT-GA-P55M-UD2-F11.aml, leaving it on the desktop, then running MultiBeast 3.1, checking the UserDSDT install and System Utilities check boxes. The installer...