It appears that if a tag is not set in an audio file, Cross cannot create it - however it can manipulate existing tags.
For example, I cannot add a record label 'Influenza Media' to a track, however if I create that tag in another tag editor (such as MusicBrainz) I can set, and change the tag with no problem in Cross.
It appears to be a problem regarding any tag, if a Comment didn't exist on the original file Cross cannot add it. Perhaps this is on purpose to keep compatibility with the original tag version? (I use ID3v2.4 and not 2.3)
Hope this helps.
3-hour radio show, Mixvibes Cross only! http://soundcloud.com/stealthii/isa3 MacBook Pro 13" Summer-2011 16GB RAM, Vertex-III SSD OSX 10.7.3 Ecler Evo 5 Mixer Technics 1210MK5 x2 Ortofon S-120 OM carts x2
EDIT : - took a 1000 of files and cleared every comments tag with MP3tag - open Cross, import them - all comments are empty as expected - took a hundred of them and tagged their comments tag with 'Cross' - quit Cross and going back to MP3tag and they was there written Cross in the comment tag. (all of them v2.4)