Harvey had constipation problems, too, and Tailer's starting to have them occasionally. Neither one of them will eat the hairball remedy willingly either. (It figures that Forest is the only one who will take it willingly, and he doesn't have any problems with that.
) Neither one of them would eat the pumpkin, either.
My vet taught me to put the hairball goop on my finger, then to swipe my finger on the roof of their mouth. They don't like it, but it gets it over with quickly and is less messy and less stressful than wiping it on their fur (which most cats HATE, and which Tailer's figured out how to wipe off on the furniture).
A tiny bit of canola oil in their food can help loosen things up a bit, too.