Are you using Back|Track 3?
If so, and if all you want to do is crack your WEP (and not learn), you can use SpoonWEP which is pre-loaded on BT3. It's a very nice tool created by a member here. There is an updated version, which is even nicer, called SpoonWEP 2 as well.
I understand if you're just trying to learn though. Other members should be able to assist you more than me.