I've just been through this. I was given 20 minutes to do a smokey eye look and the products that were available were limited. For example, we had only two types of foundation to choose from, Face & Body and Studio Stick. I found that really difficult.
For liquid products, such as moisturiser, liquid concealer and liquid foundation, squeeze a little out onto a glass dish or the bottom of a glass pot, (one should be supplied), and take it from there with a clean brush and apply it. For solid products in a compact, like Studio Tech or Moistureblend, take a sample out of the compact with a spatula, put it on the glass dish and use it with a brush in the same way.
Don't worry too much about mixing foundations, you won't really have time to faff about like that. Work quickly, cleanly, and talk your model through everything you are doing. Wipe off shadows before using them with a dry tissue, sharpen pencils, use disposable wands for mascara and lipgloss and NEVER, NEVER double dip (i.e. put a used wand back into a product). Lipstick is sanitised by twisting it up slightly and then dipping it tip down into alcohol, then wiping off with a clean tissue. Don't turn the lipstick the right way up until you've wiped it off, so you don't get alcohol down the lipstick and into the tube itself.
If you need to reuse a brush for any reason, wipe all the colour off onto a clean tissue before picking up the new colour. In order to save time try to have several eye brushes etc, so you have to waste as little time as you can manage wiping them off.
A good tip for mascara is to take a disposable spoolie, dip it into the mascara tube then use a clean tissue to press against the end to bend the brush part so it's a right angle to the handle. Use this bent brush to work upwards and downwards rather than trying to hold the mascara wand out to one side of your models head. (I hope this explanation makes sense! Please ask if not.) This allows you to get up against the model's lashes more easily, and makes it less likely you'll poke her in the eye/smear mascara everywhere. It also, cunningly, makes it impossible for you to double-dip your mascara wand. It's a tip I picked up from one of the Pro team at an event, and it works well for me.
Expect to have limited colours/types of products available and be able to choose alternatives. For example, I would have liked to have used Hepcat, Black Tied and Shroom. None of those were available on the product stand in my interview so I used Plum Dressing, Nighttrain and Phloof. Give some thought to the colours you use and try to pick those that will accentuate the colour of your model's eyes etc. You'll be asked to talk the examiners through your look and it's a good idea to have a reason (based on observing your models colouring and features, and what you've been asked to do), why you went for the colours that you did, and why you applied them as you did.
A common MAC scenario is to ask you to do a day look and then turn it into a night look. It's also not unusual for them to ask you to do a look from a look book, so it could be worthwhile taking a look at the looks from the most recent few collections and making sure you could make a good attempt at them. Be aware of the latest colletions that have come out and how they fit with the seasons fashion influences. Be capable of talking about application techniques, (e.g. which brushes you like best, which textures of eye shadows you prefer to work with and why, how your choice of product would vary depending on your models skintone and skintype etc), your inspiration, famous makeup artists you respect. I was asked whether concealer should be applied before or after foundation, which is really a matter of personal preference and there's no wrong answer as long as you can explain why.
Don't be afraid to move your model's head around. I mostly moved around my model and it was one of the things the examiner mentioned at the end. She said it helps to move your model's head around so you can work better and see your work in different light. If one eye doesn't quite match the other, for example, you're more likely to notice it if you move your model's head around.
Other things to note... wear black. I chose a few colourful accessories that matched my makeup and wore those, which the interviewer commented on. (I went for a bronzey/orange eye look and wore an orange butterfly in my hair, an orangey/bronzey necklace and a few thin glittery orange and bronze bracelets. I wore the bracelets on my left wrist because I am right-handed and I didn't want them jingling and annoying me, my model and everyone else during the application). I had to take my own brushes, so expect that you may have to too.
Whew, this is a bit of a long post so I'll stop here. Please feel free to ask for clarification on anything if I wasn't clear.
