I’ve got an AMD Phenom II X2 555 Black Edition, and tried to get the other two cores working, but it bluescreens right off, even with all four cores underclocked.
You got the extra cores working by overclocking?
Yes, it was the only way (that I've found so far) that I could activate the 'Turbo'.. MoBo: ASUS M4A88TD-V EVO/USB3 The manual is in Japanese (of course).. I guess I could look up the online version ;) What I had to do was boot into BIOS and allow it to use the AUTO OC.. I then saved the profile for future resets.
I was actually able to tweak it once (with factory RAM settings and speed.. but I had a power outage that screwed it up.. I haven't been able to do that again. (I prefer normal speed over OCing.. less problems and longer use)
BTW, what MoBo do you have.. I will look up some info and see if I can find something to help you out ;^)