you will find the memory-related chapter from Dell's instruction manual for Precision 490 - I guess that's your model. When inserting memory you should follow the directions precisely, especially pay attention to the slot numbers. Please let us know how this went.
Now, to check whether your system recognizes your new DIMMs, simply remove all other memory and put one new DIMM in slot 1. If the BIOS won't see it - and the computer won't boot - that means that either the DIMM is defective, or it is not suitable for your motherboard, or you may need a BIOS upgrade. I don't see any other possibility.