If you used the packages off the Slackware extras cd, it will install them in the proper place.
If you got the correct drivers from DRI, then the installation script will put that stuff in the right place. Go here for documentation: http://dri.sourceforge.net/
The DRI should put the libGlide3.*.so in the proper place. But if not, you need to get it from their site, and put it in the '/usr/lib' directory.
In order to get X to work right you will need to go to the XFree86 site, and read this: http://www.xfree86.org/current/DRI.html
and this: http://www.xfree86.org/4.2.1/glide.4.html
Go to these sites, read the documentation, try it yourself and then if you have problems, come back and post the problems.