I learned the first words for the force shout, as we're all supposed to in the main game. Then when we killed the first dragon of the game, I got its soul and was able to use the shout.

Since then I learned the words for fire and ice in two places, and have killed several dragons after doing so, and absorbed their souls, but it's not doing anything and keeps them both unavailable.. How do I get to use the fire and ice shouts after learning the words?

You have to spend dragon souls in the menu to activate the words. On Xbox it's the X button.

I think you also have to do the Greybeards' Summoning quest as well. I had unlocked a couple words and snagged a dragon soul but I couldn't spend the soul point until after that mission. It kept telling me "you need Dragon Souls to unlock shouts", even though I clearly had one.

If you're on PC and have remapped keys, there's an issue. See the PC bugs forum. Make sure you use the Z key to activate a shout. The Greybeards will train you so you need to see them to use them as someone said.

yeah I've been a litttle confused with this too. I've killed many dragons and have all 3 words of the force shout after going to talk to the greybeards. But it's good to know you have to spend the souls you absorb, it just doesn't unlock words autimatically like i assumed.

I havn't done the greybeards quest and were able to unlock them, I just have to mouse over them and press "R" (it shows at the bottom left when you do).

With this interface, it's hard to tell what's a bug and what's poor design.