It should be fairly large, quarter sized. Sometimes they're covered in a plastic material and stuck in odd places, where two leads feed it to the board. It could be anywhere, I can't tell you where, specifically.
It is located under the panel which has the designed for Windows XP / Vista and processor stickers. It would be called the "top cover" for my HP laptop, but I don't know what Acer calls it. Acer's user manual calls the casing on the back side of the LCD screen, "the top cover."
If you have your Extensa 4620 laptop open, you can find the CMOS battery inside the case, on the motherboard, about 2-3 inches to the right of the touch pad. You need to remove all of the screws on the bottom panel and detach the keyboard and cables under the keyboard to access the battery safely. There are also some screws under the keyboard to remove. The designers of that laptop made it extra difficult to get to the CMOS Battery.