I figured it out with the help of this wiki...
http://musicbee.wikia.com/wiki/Custom_Tags#save_to_music_file_as_tagI guess this will apply to setting up other custom tags as well, and while the above wiki helped, I'm add my process step by step to help other newbies. In this example I will be using 'comment:Songs-DB_Custom1' (Track Decade).
1. First you need to identify the 'tag code'. Easy enough as MBee has a 'Tag Inspector' built in. Right click on track > edit > Tag Inspector (top of edit menu).
2. Find your custom tag, in this example it's 'COMM/Songs-DB_Custom1', note it down and 'Close' the edit pop up window.
3. 'MusicBee' menu (top left) > Edit > Preferences > Tag (1) > Define New Tags...
4. Identifier > Enter your custom tag name, in this example it's 'Track Decade'. id3 (,mp3): COMM/Songs-DB_Custom1 (or whatever you custom tag is as noted from Tag Inspector). Save.
5. custom tags > display name > 'Track Decade' > save to music file as tag > select 'Track Decade' from dropdown menu.
6. Apply > Save > Close
You can now add 'Track Decade' to columns and your custom tags will appear.
So that's how I've got it working, but can a more experienced user confirm this is the correct procedure please?
One thing I can't figure out is why within the 'New Custom Tag Definition' does apeV2 automatically popular itself with 'Track Decade'? I only save to MP3, so have only filled in id3 field, with all the others (vorbis, mpeg etc) remaining blank... but apeV2 keeps automatically populating.