On VOCALOID3 Editor and Tiny VOCALOID3 Editor, WAV file (Stereo/WAV format) can be imported, but other formats (mp3, aac, AIFF) is not supported and can't be imported.
Please convert audio file to WAV formatted file with sampling frequency of 44100Hz/48000Hz/96000Hz, 16bit resolution in order to import it into the editor. (You can't convert audio file on VOCALOID3 Editor)
To import WAV file, you can just drag & drop the file on the editor, or select [Wave(W)] from [File] → [Import] menu.
Please note that VOCALOID3 Editor supports WAV(MONO) file type while Tiny VOCALOID3 Editor does not.