I also had this problem recently. I had some difficulty getting my Eclipse Luna SR2 4.4.2 running on Win 7 SP1 to see my newly-installed MinGW-w64 but I was able to resolve it by adding a MINGW_HOME environmental variable to my Win 7 installation, and then restarting Eclipse. (I didn’t need to restart Windows.)

In my case, MinGW-w64 had installed itself at “C:Program Files\mingw-w64\x86_64-4.9.2-win32-seh-rt_v4_rev2” and so I needed to set MINGW_HOME to “C:Program Files\mingw-w64\x86_64-4.9.2-win32-seh-rt_v4_rev2\mingw64” so that it would point to the appropriate subdirectories (bin, etc, include, lib, …).

In Win 7, environmental variables can be set via the “Environment Variables” button on the Control Panel -> System and Security -> System -> Advanced system settings. I added my MINGW_HOME variable to the “User variables” section.