After calling customer service and trying their methods of using a rubber band to grip it or using double stick tape, which didn't work, I used rubber Playtex gloves. I held the front side of the Tracker in my palm and used two fingers with the other hand to twist the cover. It actually twisted easily, both to open and close.
This was an important design flaw, to make it so difficult to open! It causes so much frustration with all their customers, and even they couldn't offer a working solution. They shouldn't have to leave it to the ingenuity of their customers to figure out a way to open it!