The first thing to check is installation technique. When you push the SODIMMs into the socket, they go in about one inch, and stop. You then have to push fairly hard with both thumbs to make them go in 1/8" more to make contact with the pins. It could be that one socket is tighter than the other, (or one RAM module is thicker than the other) and you are not getting the RAM fully inserted. This accounts for about 50% of intel iMac and MacBook memory problems.
Then, troubleshooting to isolate the problem:
Do the 1 Gb modules work individually? That is, do they both work, if you install them one at a time?
Do both sockets work? That is, if you take a known-good module in socket 1, and move it to socket 2, does it still work?
What brand and model are the memory modules?
Are they both the same brand/model?
If it is can be isolated to a single defective module, then that should be replaced by your RAM seller at no charge.
If both modules work independently, then it may be a damaged or defective socket on the motherboard, which is an Applecare issue
If both sockets and both memory modules work singly, but not when there is 2 modules in, it may be a conflict between the modules, consult your RAM seller.
