A few thoughts:
If you install into the \Program Files (x86)\ folder, you might need to activate "Run as Administrator", since Windows can be a bit protective at times. Do not install into \Program Files\ (without the x86) since that's reserved for 64-bit programs only.
Map tiles are cached (separately for each provider) for each zoom level, and Plan-G will get tiles from the cache if it can before going online. The cache is good for a little under 3 months. If you don't see any kind of map (as opposed to getting error messages), try changing zoom levels or sliding around the