HRM battery replacement troubleshooting

Hi!  I just got the HRM-Fit, and I absolutely cannot open the battery casing.  I tried using the screwdrivers I had at home, no dice.  I used coins, and nope.  I took it to the local Home Depot and got a bunch of employees to try every type of screwdriver they had, nothing.

It has a battery in there now, but what do I do when the time comes to try and replace it?  Has anyone had success opening the unit?  What did you do?  What did you use?

Thanks!

Simone