Figuring out how to open some cases is worse than the most obtuse puzzle in an adventure game. Again, your manual can help. Once the case is open, you must locate the SIMM or DIMM sockets. Assuming you've purchased your memory, look for the small circuit boards in the computer that match the size of the memory you purchased. Note that some motherboards have sockets for both SIMM and DIMMs.
Some people suggest that mixing the two causes oxidation and eventually corrodes the leads.
In a strict sense, it's true, but the chemical process is so slow that you'd have to leave the memory installed for years to make a difference. I've mixed gold and tin connectors in one system here for more than a year with nary a hint of tarnish.