1. First, download gpg files here.
 
2. After downloaded, change active directory in terminal into downloaded gpg file's folder.
- Open a terminal, then type:
      $ cd Downloads/  
   (this command to enter the download directory (jas 4 example), because the  gpg file has
    been downloaded  there).  
   after that type again:
      $ sudo apt-key add backtrack.gpg

3. Then go into terminal and type:
       $ sudo gedit /etc/apt/sources.list
    Add repository Backtrack the way copy the link below
       deb http://archive.offensive-security.com pwnsauce microverse macroverse  
       main restricted universe multiverse
    then update by typing
       $ sudo apt-get update

4. Open Synaptic and check it .. if backtrack is writing on the list (as shown 
    below), it means that you yourselves successful

5. Install backtrack your favourite backtrack apolication from synaptic or terminal ... =)