Open the gate valve all the way on the skimmer and raise it up a couple inches, that should help keep it from overflowing until it breaks in. Any chemicals, media or filter socks added to the tank can also cause an overflow like that. Keep in mind it will take 2-4 weeks for your skimmer to break in and during this time the skimmer will act erratic and can randomly overflow, I wouldn't even start dialing it in until after 3 weeks of it running in the tank.
Also, since you have no bio-load or fish waster yet don't expect the skimmer to create any skim mate for a while...you have nothing in the tank for it to skim.