RAM installation is tricky on any machine. Push "not enough to break it" and it does not work. Push "what *MUST* be too hard and it works.
Try again on install.
Also, where did you buy it and who said it was compatible? Macs are cranky about the memory DIMM features (buffers, read-delays, etc).
macsales.com recommends memory that is tested to be compatible with your year/model.
I will stop and you can answer thse questions.
See this Apple note: http://support.apple.com/kb/HT2341?viewlocale=en_US . It says if the memory doesn't meet specs or is damaged, it will just blink the light and not start up.
I recommend Crucial memory, which I've used for years on many systems with no issues. Samsung (factory fill) and OWC-branded memory, both available from OWC, also works well in macs.