Permission Denied

OK, there are a few things wrong with what you are trying to do here -- this isn't going to work the way you are trying.

Its probably not a good idea to try to simply modify your apt sources to install the bt packages -- you really should be using their installer iso/liveCD on a fresh VM. The reason for this is that the compiled binaries for bt and ubuntu are likely incompatible -- this could result in all sorts of mess for your system (crashes, apt failures, etc etc).

Secondly, the permissions for /etc/apt/source.list (and most anything in /etc) are such that non-admin users cannot modify the files -- this is essential to system security. If you were to do that, then you would need to run the command as an administrator user. to do this you would need to wrap the whole command into a single instance like so

sh -c "echo deb http://repo.offensive-security.com/dist/bt4 binary/ >> /etc/apt/sources.list"

which still would give you the same problem (permission denied). Then, having done this you would need to execute the above command as an administrator, using the "sudo" command (substitute user and do).

sudo sh -c "echo deb http://repo.offensive-security.com/dist/bt4 binary/ >> /etc/apt/sources.list"

which would duly modify /etc/apt/sources.list

Furthermore, the backtrack developers do not have the signing key for ubuntu packages (its like the certificates on secure websites -- it confirms the files come from the correct author), so apt would reject the packages until you added the bt key to the key database.

Finally, you have asked for backtrack 5, but have listed backtrack 4 in your apt/sources.list.

Thats about it -- just start with a clean VM would be my recommendation. If you get stuck, you really should be asking for support from a backtrack forum, as installing backtrack is really not an ubuntu problem.