If you see it in the Extended Tags view, it is written in the mp3.
from the FAQs:
You can also add additional file list columns for extended tag fields via View > Customize Columns... and edit the tag fields directly in the file list.
You can also add more tag fields to the tag panel via File > Options > Tag Panel.
to get DISCOGS_CATALOG it COMMENT:
Action: Format Value
Field: COMMENT
Formatstring: %discogs_catalog%
Or you might try out my web script which you can edit in such a way that the catalog number is directly written into the COMMENT field:
/t/9226/1