As you know www.Model.com previously was allowing all versions of Modelsim and Questa to be downloaded freely but it's now closed and only available to its customers.
I think they have changed the company's policy.
This is what EDA companies do. They addict you like drug dealers by freely offering then screw you up.
No problem! there are always kind peoples that satisfy themselves by sharing.
look at
http://model.com/update/modelsim-downloads
http://supportnet.mentor.com/about/reinstatement/index.cfm