You may want to read this thread as it refers to xbox and ps3 issues that you are having
http://forums.verizon.com/t5/FiOS-Internet/No-UPnP-setting-in-MI424WR-Router/td-p/513501/page/2
It also tells you how to get the the upnp enabling page if you have the newer version actiontec router firmare where it's apparently been removed for ipv4.