I cannot speak for what you do in C. What you do in Actionscript depends on what version you are using, but for any version it boils down to having a listener assigned for keyboard input when you want keyboard input to be monitored. In AS3 you would assign a listener for a KeyboardEvent and then have the event handler function determine which key was used and add it to the string. Here is a very basic implementation to demonstrate (be sure to disable keyboard shortcuts in the player when you test it in Flash using Control -> Disable Keyboard Shortcuts).
var str:String = "";
stage.addEventListener(KeyboardEvent.KEY_UP, keyUsed);
function keyUsed(evt:KeyboardEvent){
str += String.fromCharCode(evt.charCode);
trace(str);
}
