If it does not come with one, you would need to purchace the Panasonic wireless usb adapter. I think they are quite expensive and personally I think VieraCast is rubbish. I dont use it on my 42v20 as the new sony s570 has so much more netconten and the interface poos all over the viera. I feel like Panasonic just threw it in without actually thinking about any content to watch on it.
You would be better off putting the dongle money towards something like the new sony net reciever, new apple tv, a sony/samsung bluray or one of
these if you must have wireless vieracast.