How to create a flexible part in inventor dynamic simulation?
Autodesk Inventor Professional can not do some simulations... If I understand, you want to analize how will interact the belt with the tensile condition of both rollers and the weigh of the box, right?
This can be done with Autodesk Simulation Mechanical http://usa.autodesk.com/adsk/servlet/pc/index?siteID=123112&id=13773836
Check the video "Mechanical event simulation"
Take care in the material selection, probably you have to set your own material refered to the specification of the real belt.
But if you want to see the belt flexible in the simulation (in flexible movement) it can not be done.