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.
Good point. The design is pretty poor in that respect. I found if you use an Aussie 10c piece and push down slightly as you turn the cap, it turns okay.
You may still need a knife or a good fingernail to get the cap out.
Hope you have some luck.
My other gripe on the product is the non-replacable batteries in the speed and cadence sensors, but that can wait for another thread.
I got a polar with wireless HR and canance. When it came time to change the battery, they said I was supposed to return it to a service center! Lots of money, lots of time, so I did it myself, encountering the same bad design . soft materials problem. THAT is why they want you to send it to them. The O ring that seals the battery compartment also broke and Polar would NOT sell me a new one.... send it back for $$. fast forward... I made an O ring myself out of teflon plumbing tape which cost nothing and has worked better than the Polar O ring. Once you do get the back off and you want to replace it be very careful with the O ring, a little light teflon lube might help. It it breaks let me know and I can post details on making your own...
PS and I read some where that the censor battery IS replaceable by prying the censor apart. I have not had to try this yet