After selecting the folder, are the settings in "Import these file types" correct?
If the answer is yes, it would suggest that either there are no audio files in the selected directory, or that those that are in there, are already imported.
Sounds silly, I know, and I do not mean to reply to you as if you are a fool, but if you could just check the three points above, and, if there are audio files in the selected folder, that are certainly not already imported and the settings are set so that MC checks the selected folder for audio files, then there is surely another problem that will need further investigation.
Good information would be perhaps the complete path of the selected folder and the file types you expect to be imported.
-marko.