That means your "Support and Subscription Services" ( the SnS I mentioned earlier) ran out or you don't have the upgrade rights in the myVMware Portal.
If your SnS ended you can not upgrade. You have to buy a new v5 license or buy SnS for the expired license ( this will be the cost of the missing SnS time + 10% fee I think - contact your sales partner to make an offer ).
You can see your SnS in the MyVMware portal if you choose "View license Keys" and click on the key, it will show the following pic with the "End Date" at which your SnS will expire .
