Sounds to me like you didn't purchase the correct type. Especially since neither one is recognized. Have you just tried ONE on these new memory modules...I mean all by itself? Try that and place it in the first slot and see if it is recognized. Then try the other one, in the same slot, ALONE again...see if it is recognized.
The HP Website shows 512MB, 266MHz, 200-pin, PC2100 SO-DIMM as the memory upgrades for your computer.