Ok, here's fith slot TUT.
Go to the shop, buy a weapon that is able to go into 5th slot and NOTICE which class it goes to (Medic, Engineer, etc). After you have bought it, turn on fifth slot. Click on the class that is able to use that weapon in 5th slot. After you've done that, there should be a pop out box with that weapon that you JUST purchased in the box. Click on that weapon to use it in fifth slot. The fifth slot function will pop the box out as if you clicked on another slot. Example: If you click on third slot, the box pops out to the right and you chose your weapon. The fifth slot works this way as well. Turn on fifth slot, the fifth slot box pops out to the right, select the weapon, turn fifth slot off, and I believe the box closes and you can then chose weapons in other slots as normal. On = Box out. Off = Box In. That simple.