The reason is that you're using an SMBIOS the emulates a MBP with memory soldered into the board, hence no memory tab.
There's a kext you can install that can restore it but you'll have to build the kext yourself.
Update: In case anyone wants it and can't build it yourself
Hey so I am still waiting for the wifi part from Dell. I have a quick question.. When I got to my about mac. There is no memory section at all. But the about mac does show the current memory, but there is no memory section tab. Can someone help me fix this?