Sounds like either you have the wrong memory modules or they are not properly seated in the slots.
Exactly which Macbook do you have and what RAM modules did you buy?
and then > MacBook: How to remove or install memory
For quality RAM upgrades,
Lastly, this is the Intel iMac section of the Apple Support Communities and the section for Macbooks and Macbook Pros is over at > Notebooks: Apple Support Communities