I use paint shop pro, so I don't think I'll be much help. The way it allows you to change the skew is through a base-line. It kills the focus dead, though.
The way I think it's done in Photoshop is to use the measurement tool, and make a line that runs exactly along whatever you want to straighten (horizon in your case). Then go to Image>Rotate Canvas>Arbitrary>Okay. It should work.