I am trying to display and use a custom tag field I created with Mp3tag in MC without success. Updated the library from tags but no way MC sees the extended tag field
For MC to use a custom field name from a file, you have to create that field name in MC's Library first:
Tools > Options > Library & Folders > Manage library fields
Then add the field, making sure to get the name exactly right, including capitalization. It should probably be a string type field, which is the default type. When the field is in the Library, then do "Update Library (from tags)" on some files and see if the field and values transfer over. I'm sure you know you'll need to open the Tagging Pane in order to see the new field and it's value.
MP3Tag gets a lot of mentions around here, so I'm sure it's powerful and good software. You should be aware that MC is a pretty fantastic tagger in it's own right. The fact that it can use Expressions is really the heart of it's power. Being able to highlight 1000 files, type (or paste) in an expression, and have them all changed at one time, all to different values (based on the expression), is just awesome.
By all means, use the tool you like. I'm just pointing out the power of MC's tagger, in case you weren't aware. I was using an external tagger for a couple of months before I figured out how easy MC's tagger is to use. It wasn't obvious to me at first.
Brian.