Is it possible that one of the 512 mb stick doesn't work. Have you ever tried, only to put one stick at a time?
If all work, then the problem is not with the single stick.
From the manual, it says that dimm2 and dimm3 musn't be double sided and add up to total of 1 GB max. The slot nearest to the processor (center of motherboard) is dimm1 (max to 2 GB); dimm2 and dimm 3 are the one closest to the outer edge.
Surely, you can only put the 1 GB stick to dimm1 (cannot put it into dimm2 and dimm3.)
Edit -----
Double sided means having the chip on both sides of the stick, so 1 GB stick will be a double sided one. 512 MB or less will be one-sided.