First of all, it would help to have the actual error message you saw, it could many things.
Also the guide to installing Wi-Fi Radar in slackware is pretty dated (2005 in slack 10). You're best best is to try "googling" around the web for more up to date articles or discussions. I haven't tried Wi Fi Radar my self in quite a while, but I'm sure there are plenty of resources out there if you search.
I ran a quick google for "wifi radar slackware 12" and found a few links:
Podcast (looked promising enough)
Also there are alternatives like knetworkmanager which have been improved since the original thread was posted.
Hope that helps. I guess it's better than no reply.