I need a diagram of how to get inside my C521 to

Locate the cover release latch on top of the computer, near the back. Place the computer on its side, with the removable cover facing upwards. Slide the cover release latch back and lift the removable cover off of the computer. This video will show you exactly how. " />

Good day sir/maam, you need to open the side cover of the dell PC, now look at the picture below:

Turn the PC off, the encircled part is the memory slot, on its side is the release/locking lever, place the lever in the release (push it on the side) now insert the module vertically with then gently push it down until the lever snaps into lock. Do it with the next module, takes a little practice, if the module is snug and fit. thats it. Power on the PC. if you hear continuos
beeping, power the pc off, remove the modules and do the installation again. Do this several times until the PC response.

From the manufacturer's web-site:

http://support.dell.com/support/edocs/systems/dimc521/en/

under "Service Manual", click "View".

Click "Removing the Computer Cover".

Look at the pictures & instructions.

Before you install the RAM, it is important that you select
the "Removing and Installing Parts" section,
and read how/where to put your 2 sticks of RAM
into some of the 4 slots on the motherboard.

http://support.dell.com/support/edocs/systems/dimc521/en/SM_EN/cvrop.htm#wp1052950