Just use wicd; it's included in Slackware so no need to hunt down extra third party software. It connects to encrypted wireless networks flawlessly and is the easiest wireless management tool I've used.
Here's a link for it: http://slackware.mirrors.tds.net/pub...6-noarch-2.tgz
Install it as follows:
root@darkstar# installpkg wicd-1.5.6-noarch-2.tgz
Follow the instructions in /usr/doc/wicd-1.5.6/README.SLACKWARE about getting it to run at each boot. Then launch wicd-client to actually open the program.