The UPnP tab is to forward ports from a router to your computer. Use this option if peers are unable to establish connections to you.
If you do not have a router in use, you do not need to use this plug-in. If you use a router, you have the choice to use either NAT or UPnP. UPnP needs to be activated from inside your router before KTorrent's UPnP plug-in will work.
It is good to note that some users can not use UPnP because their router does not support it. If your router does not support UPnP, the only way for peers to connect to you is to use NAT.
Users without a router in their network do not have to make use of a method to establish port forwarding. Check to make sure your software firewall allows connections to the ports you want BitTorrent to use.