Backtrack is a Debian based package. So just follow the 10.04 - How do I set up an MTS MBlaze modem? - Ask Ubuntu suggested above by whitestar_99.
Those commands posted there will run on Backtrack.
And you install .deb packages by dpkg -i <package_name>
dpkg is by default present in Backtrack as it's a Debian based distribution. If not just do as in the guide.
Otherwise you can also follow Without Driver Procedure, but the GUI settings will not be same as of Ubuntu, but things would be same.