I am sure their are auto-mounting tools available for slack.. I personally don't use any, so can't help you their. But can help you in your mounting problem.
When you plug in a usb, you first need to find out what device node was given to it. Type the following command:
This will tell you where your drive is, as in is it /dev/sdc, /dev/sdb, etc.
Once you have that information, you would use the mount command as root (wait about 5 seconds after plugging in the device)
mount -t vfat /dev/assigned/to/usb /where/you/want/to/mount/it
Once you are done using it, and want to unplug it don't forget to run