First you have to get the organization tag into the MC database. Make a new library field named exactly as the tag, i.e. Organization. Then select all your flac files in MC and right click and choose Library Tools>Update Library (from tags) (any changes you have made only in MC and not saved to the file tags will be replaced by what is in the tags in the actual files). Now you can use the new field just like any other field in MC.
It is not possible to import the flac organization tag into the MC standard field Publisher. If you have also mp3 files with TPUB tags MC will automatically put the publisher information in that field. If you want to combine the data in the publisher and organization tags to have just one field that displays publisher information for both mp3 and flac files, create a new field, called Publishers, for example, with this expression:
ListCombine([Publisher],[Organization])&datatype=[list]
PS. I edited the expression above to be shown as code because it unfortunately got corrupted when it was just plain text.