You should read up on WEP and WPA cracking (there's a sea of info on the subject) and not just blindly follow the commands in the tut. You have to understand what you are doing.
Now, after you enter the airmon-ng start wlan1 command you an see in the output you've created a new interface in monitor mode (mon0) you should use that interface and not wlan1. Also you would benefit if you set the channel for the new interface. (man airmon-ng)
Suggestion - get comfortable in linux, learn the basics, read through http://www.backtrack-linux.org/forum...ad-thread.html