You first need to set linkage for the library symbol, you can do that by right-clicking on the symbol in the library and choosing properties.
Next you check the "Export for ActionScript" checkbox and ensure that the "Export in frame 1" is checked as well.
You'll see a input box labeled Class. That will default to the name of the symbol, but you can change it to whatever you want. That is the class name that you can use to create an instance in your code.
Here is how you would do that :
// if the class name of your symbol in the library is play_button
var playButton:SimpleButton = new play_button;
addChild(playButton);