I bought one of those a couple years ago - we couldn't get it to work reasonably either. Overpriced and not nearly as good as a heavy sharp knife.

I have always used this kind as did my mom and most everyone else who cut off corn that I knew. Works well for me.

The best tip I have seen about cutting corn is from Cook's Illustrated; is to cut the cob in half, that way you can rest it firmly on a board.