To answer your question directly, don't push play! AE can't play a thing because it was never designed to. Do a RAM Preview by hitting zero on the numeric keypad.
And since you're brand-new here, there's a VERY crucial question to ask: have you ever used After Effects before?
The reason: it's not the kind of application you use for a couple of tricks with and expect things to turn out right the first time. It takes a good knowledge of the AE basics to make it do what you want.
So how do you get the basics? You go here: http://blogs.adobe.com/toddkopriva/2010/01/getting-started-with-after-eff.html.