Not a bad start; could use work

It's an alright start, but I think you should've spent more time on it before submitting it. I also don't think you should've named it "PONG Online" if there's no online option available yet.
I think a better name would've been "Classic Pong", because that's pretty much what it is. Not to say it's bad, but the name is misleading.

I've noticed quite a few problems that could be fixed. I don't know ActionScript yet, but I've coded in enough languages to give you some applicable tips and tricks.
Firstly, I noticed that the ball bounces off of the paddle the same no matter what. What I mean is that the paddle's velocity or previous location don't seem to be a factor in the ball's speed or angle. A simple solution would be to periodically record the paddle's Y coordinate (since the X is constant) and compare it to the current Y coordinate. So, every frame, a variable would be updated with the Y coordinate of the paddle. When the ball hits the paddle, the ball's Y speed becomes negative plus (or minus, not sure while typing) the difference between the paddle's current Y coordinate and the previously recorded one. Experiment with this idea and tune it until you've got something pretty stable. This'll add to the strategy of the game more so than just hitting the ball wherever it flies.
Another problem I noticed was one I'm sure you've noticed with the trophies. I'm sure you're using booleans, so I don't honestly see why you'd have a problem with the easy trophy being removed upon receiving the medium one. I don't think you're saving it or you may be using a local variable instead of a global one (or however it works in Flash, sorry if I'm off a bit). As for the wall achievements, you should just set the achievements according to the highest score. That way you only need one variable for all of the achievements. Nifty way to handle it.

Those are the improvements, here are some additions.
I'd like a mute option. I really think this is just something all games should have. Since options seem to be occupied by achievements, however, you should probably sort that out and make achievements an entirely different frame. It'll be pretty crowded once you add actual options.

Good luck with your game man, if you want some more suggestions just ask in a PM or something.

good

but you need to be careful about the use of pong name. ever hear of plasma pong ? it had fluid colour fx+ jet controls it was a great game but the makers of original pong had it removed just because of the name and simularity to pong. http://www.plasmapong.com/

okie dokie

pretty fun
-WINNING

Welp..

Well done as I assume you're just entering the world of coding? Normally I'd bash someone for uploading something so basic, but it does sound like you're just starting, so, again, good on you!
I'm not familiar with actionscript so I can't make any specific calls, but one small issue I noticed on playing for 5 seconds was that my cursor was visible as a text input type of cursor.. so.. I'm not sure what's going on with that, but you might want to look at it. I also notice that the opponent (1-player) paddle seems to jitter occasionally when moving. I assume this has to do with your basic AI... the paddle is just following the balls y-pos.. but there should be some way to tween that out or something.. I dunno.. fiddle with it?
How about continuing the game instead of quitting after one win/lose? (Or do you not understand variables yet and the ball-movement code is cut n' pasted?)

Not to sound like an advertisment here, but if you're interested in coding with quick, visual results in a high-level language that has plenty of power, have a look at one called Processing. You might find it superior to Flash.

Seems good, but incomplete.

I'm going to rate this accordingly since it is unfinished.

However, I do like where it is going.
I noticed one glitch; I got the wall achievement, then the medium achievement, then the easy one. Upon getting the easy one, the medium AND easy disappeared.
I'm putting this here because you may want to look into that, also it seems I can forever scroll to the left or right in the achievement screen.

The idea seems like it could work out very well, I'd love to play someone else online.

As I said before, the score I give is my impression of the game in it's current stage and where it can go. I would willing score it higher once it is finished and if it works as planned.

Hope all goes well for coding it!