From your description, I'm guessing that you're talking about the battery in the wearlink transmitter (that attaches to the chest strap).
It is a stupid design, with the points of maximum leverage (the outer edges) having least depth to the groove, and with it being made of soft plastic it could have been designed to round off!
The best bet is to use a coin which matches the curvature and width of the slot as closely as possible. If it's already been rounded off though then that still might not work. Failing that, try a wide flat-head screwdriver and press down HARD into the slot as you try to "unscrew" it.
One thing to be aware of though is that the cover doesn't have to rotate very far in order to be removed (only a few degrees, as per the arrow on the back). It's a very snug fit and won't just pop out when you've unscrewed it far enough, so you might not realise that you have rotated it far enough. You will probably need to get a fingernail or something very fine to prize out the battery cover.
Best of luck.