Greetings! Sorry to open up such an old topic, but I had the same problems as you, and could not find a fix. In earlier versions of MP3Tag, I could easily drag and drop from an explorer window into the main window of MP3Tag and it would allow me to load up the song (or multiple songs) and edit as I wished. However, since upgrading to Windows 7 I've run into the problem that I can no longer drag and drop into MP3Tag.
I have found a solution, however, that has not failed me yet! Simply follow these steps:
1.) Navigate to the folder where you installed MP3Tag and search for "Mp3tag.exe". Usually it will be: C:\Program Files\Mp3tag .
2.) Right-Click on "Mp3tag.exe" and select PROPERTIES, then select the SECURITY tab.
3.) Click on the EDIT button, then click on the ADD button to add another permission group.
4.) In the input box labeled "Enter the object names to select..." enter this, exactly as you see it: Authenticated Users , then hit OK.
5.) In the next window that appears, check the FULL CONTROL / ALLOW check box, which should be under the highlighted "Authenticated Users" group, then hit APPLY, then OK.
6.) Exit out of all windows, then start MP3Tag once more, with the ability to drag and drop MP3 into the main window to edit!
I hope this helps someone! Thanks for your time.