Go to http://slackbuilds.org/
then to the "how to" and read it. It will show you how to download
and install chemtool, if you will install chemtool by the article you can install most anything
at slackbuilds the exact same way. When you find a program that you like read the readme and you
might have to install a couple of dependencies before you install actual program.
There are several different ways to install programs in slackware, another way is "sbopkg" which
is very easy.
Another way is to go to http://www.slackware.com/~alien/slackbuilds/
and download pre built
packages, just download the tgz or txz package for the version that you have. then