It's not a matter of "telling" the AI or not. If the AI is sufficiently intelligent, it should be able to observe that its computational resources are bounded, and infer the existence of the box. If it can't make that inference (and can't self-improve to the point that it can), it probably isn't a strong enough intelligence for us to worry about.
If you implement the AI in a universe with discrete time, you can simply lengthen the amount of time it takes for the real-world computer to calculate the next time-step, without alerting the in-universe AI to any lag. This has the added benefit of allowing us enough time to unplug the AI should it become malicious. See my comment here, which may be related.
Boxing an AI is the idea that you can avoid the problems where an AI destroys the world by not giving it access to the world. For instance, you might give the AI access to the real world only through a chat terminal with a person, called the gatekeeper. This is should, theoretically prevent the AI from doing destructive stuff.
Eliezer has pointed out a problem with boxing AI: the AI might convince its gatekeeper to let it out. In order to prove this, he escaped from a simulated version of an AI box. Twice. That is somewhat unfortunate, because it means testing AI is a bit trickier.
However, I got an idea: why tell the AI it's in a box? Why not hook it up to a sufficiently advanced game, set up the correct reward channels and see what happens? Once you get the basics working, you can add more instances of the AI and see if they cooperate. This lets us adjust their morality until the AIs act sensibly. Then the AIs can't escape from the box because they don't know it's there.