The instructions you provided didn't match what I saw in the developer console. I didn't have register but it did have Credentials. I clicked on that and create new key and then browser key. I used the API key that it created. I plugged that into your plugin and it appears to have worked. The Google id and key stayed put when I saved. However it isn't importing anything from Google+. I created a Google+ category and have Maximum number of new activities to import set at 100. I enabled comments and meta tag.
I clicked on Manual mass import and it said successful almost immediately but nothing imported.