I too bought a ME 109 ARF that was already completed. But the guy I bought it from was kind enough to provide the manual as well. So - here goes from the manual.

Model Tech ME 109 .25 Size

The CG is located 2-1/2" back from the leading edge of the wing at the fuselage sides. This location is recommended for initial test flying and trimming. There is a 1/4" margin forward & aft, but it is not recommended that the CG be located any further back than 2-3/4". BALANCE THE ME-109 WITH THE FUEL TANK EMPTY.

Using a couple of pieces of masking tape or a pen, mark on each side of the TOP of the wing 2-1/2" back from the leading edge (you balance the model inverted). YOU KNOW THE REST.

I have a Saito .30 in mine with a 10x6 Master Airscrew and it flies great with the CG right where Model Tech recommends. Be careful on initial take off it will want to dip a wing if you don't let the ground speed come up, but once flying its pure fun. Mine likes to land a bit hot but nothing unmanageable. No unexpected tendencies that I've found yet.

I'm sure you'll have a blast.