Hey Steve,
I has a similar problem in my game. It turns out the keystone woks the same for all 4 ruins that exist. There is one main ruin where you will find 3 more keystones and map markers for the other 3 ruins. However, if you happen to find another ruin first (like you and I did) you use up your keystone and are stuck in the quest. So you are supposed to first go to the temple mentioned in the riddle (near Furtsanden (sp?)).
If you can't go back to an old save, you can just cheat to give yourself another keystone to get into the first ruin. Use the additem console command to add 00025B33 to your inventory.
Also, i had to play around with the quest stages a bit too (I looked them up using the construction set) before I got everything back on track. Anyway, hope this helps.