At present, the USB port has no use at all. I've tried to hook a keyboard to it (as some have done with the 721), but no luck. It's been rumored that a future upgrade will enable you to upload MP3's to the receiver with a USB flash drive; but that won't be until after we finish beta-testing it.
522 PIP mode has nothing to do with your TV's PIP mode; 522 PIP is independent of TV PIP. You have to use one or the other:
522 PIP Mode: Switch your TV to "single mode" (read the sticky in this forum). Then you'll use the remote's PIP and Swap buttons. Tradeoff: If you have TV2 in another room, that TV won't work until you activate "shared view" (also in the sticky), and then TV1 and TV2 will get the exact same programming.
TV PIP Mode: Simply connect your TV's two inputs to TV1 and TV2. (If TV2 goes elsewhere, you can split the RF output *or* connect the PIP TV to the RCA jacks.) Then use your TV remote's PIP controls. Of course, the second picture will be whatever's on TV2.