link for you
http://help.nowtv.com/article/Setup-NOW-TV-Box
Apparently there are either white or black boxes, the link above refers to the 'white' although there is a link there as well for the 'black' one.
I set up a friends set (black) as per the instructions supplied and used the HDMI cable to connect box to tv. Also connected ethernet lead from Broadband to NowTV box. Leave the aerial cable connected to the TV to enable Freeview, should you so desire. Don't forget to use the 'Source' button on your tv to find the NowTV box input signal.
I have no idea about registration, but I suspect you would have to register first and that the Voucher would commence immediately.
I've reported my earlier reply, because it was total nonsense. Apologies, must be brain fade after Xmas.