I have an ASUS M4A79T mobo and the CPU I want to put in is an AMD Athlon 64 x2
When I put it onto the socket, three corners seem to fit fine, but one doesn't...
I have the lever up, and I matched up the triangles on the mobo socket and CPU
The two parts are incompatible. The M4A79T is Socket AM3 and has 938 pins. No Athlon 64 X2 will fit in that socket as they are Socket AM2 (940 pins) or Socket 939 (939 pins). Even if you jammed it in there, Socket AM3 uses DDR3 and the DDR or DDR2-only Athlon 64 X2s would not work.
You will need to get a Socket AM3 Phenom II (any Phenom II except the 920 and 940) or Athlon II for that motherboard. Or, you need to get a Socket AM2/AM2+ or Socket 939 motherboard and some different RAM if you want to use the Athlon 64 X2.