I'm trying to get my mind around A/C/E. Are they essentially the same things as skills for PC's but just undiferentiated? Does the GM tell the players, "okay, this foe has 3 aggression, 4 cunning, and 2 expertise" or does the gm just use throw in the dice as he (or she) see's fit with no explanation?

You can describe it any way you see fit. Personally, I say it out loud most of the time. Like, "Here's his dice pool, I'm using an aggression from his pool to add a fortune die". I use them to simulate skills, like when there's an opposed check where dice are added if the opposition has a certain (relevant) skill or specialization. I also use them in combat to reflect stat mastery or skills. Occasionally I'll use them to perform manoeuvres. I'm sure there are other uses for them that will crop up as we play.

It's a very flexible system that not only allows GMs to stat monsters on-the-fly, but also makes game prep shorter as you don't have to design every monster detail before the game.

If you have time, then you might like to stat out key NPCs.

Given them Expertise dice and Fortune Dice in Weapon Skill or whatever works very well in my experience.

I tend to make the description a bit more general "The orc snarls at you, its eyes full of hatred, putting all of it hate and strength behind its swing with one thing on its mind; to remove you head from your body".... or alternatively, "the orc stands back wary of becoming too exposed to your attacks, it seems far more interested in its own preservation than in disembowleing you,... for now." If i am rolling the dice pool in front of the players, i will then justify that the 2 or 3 fortune dice are due to his agressive action, but if I'm rolling behind the screen, i won't explain exactly what dice I'm rolling.

I think the players need some kind of clue to what's going on though (hence the description) so that they can decide whether to use active defences or not.

IMO, its a bit cruel to add in aggressive dice without at least giving some hint to the players that it might be a good idea to use some active defecnes, if they have any available!