Cracking WEP the simple way with Backtrack 3 - SpoonWEP

After your card is in monitor mode & your mac address has been spoofed, you'll run the following.

airodump-ng ath1

You'll want to copy the AP's mac (bssid) and paste it in the Victims Mac field.
Select the channel of the AP. You can leave the injection rate the way it is, & select the ARP Replay if it's not already selected. A konsole window will pop up titled WS DUMP. SpoonWEP will display the key at the bottom.