After downloading vSphere 5.1 successfully I tried downloading vCenter and received the following error:
You either are not entitled or do not have permissions to download this product.
Check with your My VMware Super User, Procurement Contact or Administrator.
If you recently purchased this product through VMware Store or through a third-party, try downloading later.
This is what I got the first time I tried to download vSphere 5.1 and after going to your link I had no trouble. Seems that download is acting up or something.