Do your files have the ".midi" extension?
VOCALOID4 Editor cannot open files with the ".midi" extension.
Change the extension of your standard MIDI files (SMFs) to ".mid".
In addition, if your files contain system exclusive (SysEx) data, VOCALOID4 Editor may not be able to recognize and load those files correctly.
If you are unable to find a solution in the Troubleshooting section, please use the Inquiry Form to submit an inquiry.