Please make sure that ReSharper loads the MSpec plugin successfully. You can see loaded plugins in ReSharper/Options/General/Plugins.
Also make sure that the MSpec unit test provider is enabled. Its in the options dialog, at the bottom of the tree to the left.
Your contexts don't need to be public to be detected. A class with an It field is all that is required for the plugin to detect tests.
If that doesn't work, please try to run your spec assembly with mspec.exe (command line runner) - does it execute specs correctly?