Hi nick021, and welcome to LQ!
Ok, plug your pen drive into one of the USB ports - whichever one you like the most! Then, go into terminal/console. If you are using GNOME, then just right-click on the desktop and select "Open Terminal". If not just browse your menu for Konsole or a terminal program and open it. Then, type in the following:
# mount -a
That should automatically mount your drive (it mounts everything visible), and the mounted drive should be found in either /mnt or /media (I'm not too familiar with Slackware).
If you are still having trouble mounting it, or trouble with file permissions, then please reply stating the problem.