Install Windows7 into Bootcamp (using mid-2011 iMac)
You have not burned the Windows 7 ISO file to the DVD-RW. For example, I have the Windows 7 ISO file named English_Windows_7_Professional_SP1_W64_X17-59186.iso. To burn to a DVD-RW, I would need to right click on the file in the Finder application and select Burn Disk Image "English_Windows_7_Professional_SP1_W64_X17-59186.iso" to Disk..., as shown below.




  Note: The Windows 7 ISO is required to include Service Pack 1 (SP1). See the website, Install Windows 7 on your Mac using Boot Camp, for more information.


Just so you know, 64 bit Windows 10 runs great on your model Mac. I know this to be true because I also have a iMac (21.5-inch, Mid 2011).