I can only find one slot for a RAM module, under

I can only find one slot for a RAM module, under the memory bay door on the bottom of the machine, but the Crucial site says there are two slots. A system info readout for it says I have 512 of RAM but the module I find is only 256, so I suspect there's another module somewhere but don't know where. I want to upgrade the to 2048 using two 1024 modules, but won't order them until I know where to find the missing slot. Help!