UPnP is nothing but in simple(layman) sharing files over a network.
Steps to follow:
1) Enable UPnP services in xtreamer.
2) Download PS3 media server from the net and install it in your laptop or PC.
3) Setup a network (wired/wireless) between xtreamer and the laptop/PC
4) Run PS3 media server on the system.
5) Go to media library in xtreamer and select UPnP
Now you can play your harddisk content in xtreamer.
Universal Plug and Play (UPnP) is a set of networking protocols for primarily residential networks without expert administrators that permits networked devices, such as personal computers, printers, Internet gateways, Wi-Fi access points and mobile devices to seamlessly discover each other's presence on the network and establish functional network services for data sharing, communications, and entertainment