Hey,
You can find the cmos battery on the bottom of the motherboard. To access, you need to remove almost all part of the computer: keyboard, top cover, display, cd-rom, hdd, etc. You need to remove dozens of screws, most of them are philips head, some of them are hexagonal. It's pretty lot of work for a small thing, i have never seen such a bad designed computer. They could simply put this cmos battery under an easily removable cover, but they decided to make the customer's and repair worker's life difficult.... It's awful...