Per this [1] info sheet, it looks like the RAM you bought should work. The most likely issue I can see is that you have the RAM sticks plugged into the wrong slots. It looks like you need to have the ram sticks in the slots with the white retention.
If the above still does not help, try the slots with the black retention, just to be sure.
If all of the above does not work, try one stick of RAM in the white slot closest to the center of the motherboard. If that doesn't work try the other.
If all else fails, we need to find what beep code the PC is emitting. Take a look at this [2] site and see if you can decode the beeps.
Let me know what you find!
[1]
https://www.dell.com/downloads/global/products/optix/en/desktop-optiplex-960-technical-guidebook-en.pdf
[2]
http://www.dell.com/support/article/us/en/04/sln143065/dell-optiplex-and-vostro-beep-codes-and-error-messages?lang=en