I want to purchase movement trays which I can use to magnetically attach removable 28mm figures onto for use in wargames.

I purchased a skirmish tray (GMT247) and circular flex-steel base bottoms from you, but apparently this particular skirmish tray isn’t set up to be used with magnets or flex-steel.

See attached (or use this URL: http://portal.roguecomet.com/wp-content/uploads/2017/09/litko-question.jpg).

Question: I’ve heard people say they’ve bought skirmish trays and attached removable, magnetic 28mm figures to them, so what parts or items would I need to buy from you to do this?

Most customers use the magnetic base bottoms on the actual figure bases, then use the flex-steel base bottoms as inserts in the trays.

Your half way there, you can use the base bottoms you have and stick them into the tray. You just need to order the magnet base bottoms for your figures. Looks like your using the 25mm circle bases.
http://www.litko.net/products.php?product=Circular-Miniature-Bases%3A-25-mm
LITKO Game Accessories, thousands of ways to upgrade your games!