Do you see the pin labeled "key" ? There shouldn't be a pin on the connectors coming from the case on there, or it should be blocked off. And on the motherboard, there shouldn't be a pin there (will be a hole there). Just line those up and you're good.
As for the memory error, run Memtest86+ for a minimum of 4 hours. Memtest86+ - Advanced Memory Diagnostic Tool
Download the ISO, and burn it to a blank disc, and boot off of the CD. IF you don't have ISO burning software, use ImgBurn: The Official ImgBurn Website
Both programs are free.