You can't adjust the collection cup height until it's in the water and plugged in making foam. You'll move the cups up or down to allow dry or wet skimming, respectively.
To be on the safe side, start with the cups all the way up. Plug it in and look at the foam the skimmer is making. Now start to lower the cups until the foam is going about half way up into the neck. At this point I'd leave it be for a while to let the skimmer break in.
Again, if it's a brand new tank with no fish, it really won't skim much, if anything.