The cables are corroded inside the housing. Since you already have the handle off the first thing I'd try is get a 2 screw cable luber and try to force some penetrating oil into the cable housing: Yamaha Cable Luber Deluxe At Parker Yamaha Com
Let the oil set for a day and repeat the process a couple of times, trying to work the trim each time. You also might try putting some in from the back end where the cables come through the transom.
It depends on how badly they are corroded as to whether this will work, but I've seen them free up more often than not. If this doesn't work, you can try and strip down the system further to see if you can break it free. This guide may help (scroll down): http://www.fullwavemarine.com/TIPS%2...AHA%20PWC.html
. If all else fails your only option is to replace the cables.
Once you get them free, make it a habit to run the trim back and forth a few times every time you ride the ski, and lube the cables with the luber as part of the winterization process.