1. Select multiple photos (I was selecting 13 Canon 5D Mk III CR2s + JPEGs, for a total of 26 files)
2. Image -> Set GPS...
3. Choose location on map
4. Click "Accept Location"
5. Click "Apply to All"
6. Wait for it to finish
Now you cannot get out of that dialog or close the app without killing it in Task Manager.
Windows 7 x64, Photo Mechanic 5.0, build 13119 (2b80ced)