Roger,
You can either use the Export button in the Preferences dialog and then copy the .PMX file to your new computer and then use the Import button in the Preferences dialog on your new computer and choose that .PMX file you copied over, or you can copy the com.camerabits.PhotoMechanic folder that is in your Library/Preferences folder inside your home folder to the same location on your new computer.
The Export/Import from the Preferences dialog is easier by far though.
-Kirk
