How do you prove to them you bought the game? Take a picture or something?
You have to send a photo of your receipt, as well as information regarding where you bought it etc. The process is detailed here http://ubisoft-en.custhelp.com/app/answers/detail/a_id/17223
So does anyone know how Rocksmith on PC works? Like, how would I use the Steam version? Where do you get that dongle they use to translate the sound into the game? I've always been curious about the game, but this PC version especially sounds like something they didn't really think out before doing it. This thread kind of cements that.
You can buy the cable separately if you want (though it's cheaper to buy a physical copy bundled with the dongle. However, if you already have the 360 or PS3 version, you already have the dongle and thus you can buy the Steam version without getting a new one.