Install Windows 7 on a (17-inch, Late 2011) MacBook Pro

My goal is to install Windows 7 on my (17-inch, Late 2011) MacBook Pro. The model identifier is MacBookPro8,3. The problem is the Windows 7 USB or DVD installer is not being recognized when holding down the alt/option key at startup. This is where I am currently stuck: I have tried to follow David Anderson's guide: https://apple.stackexchange.com/questions/225387/no-bootable-device-usb-2-0-macbook-pro-mid-2014/225521#225521 I get an error message when trying to load the bootcamp.vmdk file (step 7)