If you ordered replacement batteries online and they don't directly fit into your Roomba (as the guide indicates), you most likely purchased the incorrect replacement.
The Roomba Discovery, 400 and 4000 series all use the same replacement battery pack. The Roomba 500 series and above use what appear to be a shrink-wrapped battery set.
If you're certain (or curious!) that you need to open the battery pack, I would suggest starting with a hair dryer (a heat gun may generate too much heat and make the battery cells explode) and a razor blade around the glued seam.
You most definitely do not want your Dremel to slip and cut into one of the battery cells, spewing battery acid everywhere.
iRobot Roomba 4100 Battery Pack Replacement