If that is the same board as yours, search for a bios for it. Check your bios and see what version it is. Report back. If there's a newer one available, it may be necessary for the newer chip to be recognized.
Here's one I found for version 5.20 circa 2010: http://h30434.www3.hp.com/t5/Desktop-Operating-Systems/M2N78-LA-Motherboard-BIOS-Update-ver-5-20/td-p/255472
I don't know which version you have now, but I'm thinking it's too old for that processor.
You would have to re-install the original CPU and update the bios. Then change CPUs.
...and send that link to the CPU upgrade list you found, if you would.