The trial copy converts to a fully-functional copy simply by typing in the registration code. Absolutely not necessary to uninstall anything (unlike Microsoft and their goofy test drive)
I suspect that the registration number is not entered properly. iWork is known to use the letter 'O' in the license code. If you think you see a number zero, try the letter 'O' instead. I'm betting that will work for you. This also happened for me, and I thought I had a bum copy, but it was just needing the 'O'. Strange for Apple to do that, eh?