It could well be that your memory is not compatible, but make sure that you are installing these in the proper slots. The memory slots are arranged like this:
4 - pair 4
3 - pair 3
2 - your new memory pair 2
1 - original memory pair 1
1 - original memory pair 1
2 - your new memory pair 2
3 - pair 3
4 - pair 4
So, the memory must be installed by pairs from the inside out. You can skip empty slots, but the pairs have to be in a paired position. Make sure the memory is seated properly.