Originally i had corsair xms memory (DDR3-1600, 1.65v). Even though it shows supported in the MB manual to make sure it's not the problem i bought a Kingston hyperblu memory (DDR3-1333, 1.5v) but still have the MEMOK light constantly on. I did press that MEMOK button and it tried to tune the memory by resetting couple of times and eventually the LED stays lit.
2. Clean the memory chips pins and very importantly the memory socket connectors. They may have a special paste over them that prevents the memory from being connected correctly.
To clean them you could try a cotton bud dipped in Methylated Spirits or isopropylene