Add a reference to the COM assembly "NATUPnP 1.0 Type Library." Then, you can access the collection of port mappings via UPnPNATClass. Here are a few examples.
Code:
IStaticPortMappingCollection mappings = (new UPnPNATClass()).StaticPortMappingCollection; // Fetches the collection of mappings
mappings.Add(8484, "TCP", 8484, "192.168.1.101", true, "The Banana Program"); // Maps a port to external port 8484, protocol TCP, internal port 8484, internal client 192.168.1.101 with description "The Banana Program"
mappings.Remove(8484, "TCP"); // Removes a port mapping with external port 8484 and protocol "TCP"