You will want to become familiar with the following site:
This is where you can get Slackware packages ready to install using, as root:
This will easily install Slackware packages. To install from source (the tar.bz2 file you downloaded) use:
tar -jxvf nameofpackage.tar.bz2
This will uncompress the package. Ususally (read the README file), you simply then change directory into the directory that was just created (name of program),
Then, as root, type:
Keep in mind it is much easier to use installpkg in Slackware and most of the packages you will need for Slackware can be found at the above site.
Hope this helps