I did a fresh new install of Win 8.1 and ran into the same problem.
Yes, I was one of the person who struggle with that issue over the weekend. So that is why I shared it here in case someone falls into the same problem as me in order to troubleshoot the issue. I tried it out 2 times to see if there are any changes, but I didn't see any changes.
After selecting the PMDG aircraft, I did not see an activation window. But today I have reformatted my hard drive, because I wanted a clean install of Windows 8.1. Maybe you have both fsx installed on your machine, or you probably deleted the boxed version and kept the FSX Steam Edition only, which would mean that earlier SimConnect version are present on your machine, I don't know, I am just guessing.
