To open the battery door on this camera

1. Hold the camera upside down with the lens towards you
2. Slide the LOCK switch to the right and hold
3. Slide the battery door towards the front of the camera as far as it will go
4. Flip the battery door open

If the lock switch won't move, it may be stuck due to dirt, or corrosion inside the battery compartment. A drop or two (not more!) of WD-40 might help in getting it moving again.

If the lock switch moves but the battery cover won't, either the locking mechanism is broken and stuck, or the battery cover itself is stuck, the latter is likely due to corrosion in the battery compartment. Apply a few drops of WD-40 around the seam of the battery door, wait a while and see if the battery door will move. Repeat the operation if necessary. Don't force anything. If the battery door still refuses to budge, take the camera in for servicing. Same thing holds if the battery door will open but there's abundant corrosion due to batteries having leaked.

If you succeed in opening the battery door with the aid of WD-40, remove any traces of the latter with a cotton swab and some alcohol: the solvents in the WD-40 may attack the camera's finish.