MP3TAG can only add ISRC in ID3v2 tag. It can be embedded in both MP3 and WAV files, but For WAV files, there is also a special standard for embedding ISRC, that MP3TAG does not supports. (in an 'axml' chunk)
Two apps that supports adding it are:
1. ISRC Editor by Sonoris Software
2. MetadataTouch by Digital Confidence