Hi,
you can see neither of the 3 preinstalled channels? If this is the case, i can only think of 2 problems: there is no bandwidth left for the tv set or a firewall is blocking Google Desktop. Anyway, the channels on that gadget are a bit old, they may not be offering streaming anymore; there is a newer gadget called Web TV with newer channels, maybe this one works:
http://desktop.google.com/plugins/i/webtvandradio_5657.html?hl=en(Btw, you need to have Windows Media Player installed for both gadgets)
Thanks,
Teo