Are you a student?
Hi Hailey21, welcome to the Autodesk Community!
Inventor 2011 can be downloaded from our Virtual Agent linked here.
From the front page click on Download Links and then select the product and version.
Please accept as solution if my post fully resolves your issue, or reply with additional details if the problem persists.
I think the student community only issues serial numbers for the 3 most current releases.
JDMather is absolutely right.
While you may be able to download the 2011 version from the Virtual Agent, it is a 1-month trial version and you would need the serial number from the Education Community which is only issues for the current version (2015) and the three most recent ones.