Try one more time
Right click on the project file > select Open With > Select Choose Default Program > Look under the Recommended Programs field, or search through Other Programs.
Or safest yet, Browse (Browse button) to the correct program > Then tick the "Always use the selected program to open this kind of file".
You may find it directely on the Open With Menu (It don't show the extension here, so it can be a hit or miss).
Good luck. It should default to the program you choose, also the 64-bit version.
Just to make sure. You are at this location, C:\Program Files\Steinberg\Cubase 7, and not in the x86 path?