Im not so good at this too since am just buliding my own pc with the help of this forum, but from what i know, you chose the wrong mobo for your cpu, or vice-versa.
If you want a 2nd Gen Core i5 2500K CPU (Called Sandy Bridge), You need a brand new mobo, built and based on the new P67/H67 chipset.
If you want your ASUS P7P55D mobo (built and based on P55 Express Chipset), you need a 1st Gen Core i3/5/7 CPU (Called Lynnfield.)
To summarise, its either ASUS P7P55D (Or any P55 based/built) mobo with Core i3/5/7-xxx Lynnfield
CPU, then there's Core i3/5/7-2x00/K Sandy Bridge CPU with one of those mobos
or any of H67 ones.