A slax module is either .lzm or .mo. A slax package is .tgz.
You would have to extract the tar.gz file and then read the INSTALL file within the extracted folder for installation instructions.
However this can be a problem if you don't have the necessary compilers and/or libraries installed.
Ideally it would be better for you, being new to linux, to find a slax package. I would however assume that the drivers for your card are already available in BT3.
Possibly all that is required is some minor tweaking of the /etc/X11/xorg.conf file.
Edit: I've just taken a quick look... and it doesn't look like there is a slackware package or module available on the internet... but I could be wrong.
If BT3 doesn't have that driver already... then you will have to compile that source package that you downloaded.