Had factions, downloaded nightfall, still can only play factions.

The title says it all, i've had factions for months and just developed a craving to be a dervish. So I used my Nightfall and tried having them both installed, but when i (attempt to) make a new character it still says I can only use the Factions campaign

Have you entered the Nightfall key?

i don't now where to enter it, it asked last time. srry if i'm a n00b, no comp experience (obviously)
edit: found it, thanks!

Under where you enter your password look for the I have a new key option. Follow its instructions and you will be playing Nightfall in seconds.