It doesn't look like the 790gx-g65 supports ACC (advanced clock calibration) that lets you unlock cores without a BIOS update. Which version are you on? MSI 790GX-G65 ATX Motherboard Review
Looks like BIOS ver 1.5 or later is required. Latest BIOS version is 4.1 MSI 790gx-g65 website
Read the note about using LiveUpDate if you're going from ver 1.xx to ver 4.xx.
Thanks for the help. I downloaded and flashed bios with lastest version and unlocked the 4th core.
Now i just have to make it stable when running the stress test. I have to figure how to enable the manual voltage for the core and bump it a little.