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After filling out the form, I only see a download option for 32-bit.
In order for Navisworks to view Inventor files (IPT, IAM, IPJ), an inventor product must be installed on the machine. Navisworks for Manufacturing installs Inventor View 2010 along with Navisworks 2010 to allow this. You could install the 64-bit version of the product trial of Architecture, Engineering & Construction Navisworks Trial and then install Inventor View if you don't have a 2010 Inventor product already installed on your machine.
Inventor View 2010 is available for free here:
http://usa.autodesk.com/adsk/servlet/pc/item?siteID=123112&id=12624908