Trouble installing upper headset cup.
 

The cup keeps wanting to go down on one side and not the other. I'm worried the head tube isn't perfectly round and this is causing the problem.

Recommendations?

How are you installing it? You really need to use a headset-cup press to apply very even pressure all the way around the cup. Long threaded bolt, big washers and a nut from Home Depot also works very well.

http://www.davemuggleston.com/bmx/bearingcuppress.jpg

or headset tool kit:
http://cdn.mos.bikeradar.com/images/...xvn-670-70.jpg

You can also do it with some adapted spacers and an bench vise (if it opens wide enough).

One thing to be aware of, you can only press one race in at a time, otherwise both races will cam over to one side. I prefer to press the conical upper race since that's the stronger configuration, then I press in the lower cup. For most headests it's important that the press bears on the inside of the cup directly over the section that goes into the tube. Pressing on the lips of the cups can crack or distort them. (That's what those stepped sleeves on the Park press are about.

Having always just used a block of wood and a hammer, I'm no stranger to this.

IME, you just need to give it enough of a whack to jam in (not more than ten degres or so off), then you can tap away making it level-ish and work it in.

Don't even think about it with a carbon headtube, though...