Page 14 of the instructions says to bolt it to the firewall with 4 socket-head cap screws, lockwashers, and flat washers so it sounds like the holes are predrilled and have the blindnuts installed already. If the blindnuts are not installed, you just stick them into the holes from the tank side so that the barbs bite into the firewall.
To mount the engine to the mounts themselves, if they are the glass filled (black nylon) mounts, your best bet is to drill and tap for 6-32 cap head screws. I do this with every plane, whether ARF or kit built and have never had one fail. I have had one mount break where it was drilled but that was during a full throttle hole dig and the broken mount was the least of the damage.

I have never had a Great Planes ARF but have built several of their kits and the motor mounts they use are very easy to install. A VERY useful tool to have is the Great Planes dead center mount marking tool thingie. It consists of a tapered tip that centers in the engine hole with a drill bit that slides down thru the holder so you can pre drill the mounting holes. A must have tool, in my opinion!