There are some questions that need to be answered because you didn't tell us everything.
What CPU model do you have? You said it's an i7 so is it Sandy Bridge or Ivy Bridge?
What did you have for ram before you bought this 16gb?
Do you have the ram in the correct slots on the MB according to the MB user manual?
What is the brand and model of the ram that your trying to install?
The ram that you need to have for that laptop is DDR2, so if you have DDR3 ram it won't work.http://www.cnet.com/laptops/alienware-m15x-intel-core/4505-3121_7-33772443.html