Have you received a new remote control and text to activate your 360 upgrade, only asking as you didn't say. If you haven't received the remote control or any info saying your upgrade is on it's way, then it looks like your upgrade has not been processed and that is why the upgrade does not appear in your apps. If it does appear in your apps I wouldn't do it until you have received a new remote and activation text. If you have got the remote and text, but no upgrade app then the only thing to do is phone virgin and select the "I am thinking of leaving option" and hopefully they will put you through to someone who can help.
I believe the link is or should be in the apps, if it isn't there you will need to speak to someone at virgin in the 360 team, phoning and selecting the "I am thinking of leaving option" should get you put through to someone who can help.