I have an Acer travelmate laptop too, i never had to change the CMOS battery on it. But if you are wondering which part to open, i've heard that its the big rectangluer box, in the middle of the laptop underneath. be careful whilst attempting to do it.
check out my post on how to install memory into a laptop, that is basically the same model, that will at least get you started in opening the case. but as gman said changing the cmos battery can be a daunting task as it most likely will not be easy to get to.