You can buy microwaveable macaroni and cheese packs if you're a fan of powdered cheese filled with all kinds of preservatives. If you're looking for a few bites of mac and cheese that uses actual cheese and milk you can get your fix using a coffee mug and a microwave.
Parenting weblog shares the technique: add half a cup of water and a third of a cup of pasta to a coffee mug and microwave on high for two minutes and stir. The water might overflow while heating -- this is OK but use a larger container if you wish to avoid. Repeat the two minutes on high and stir again; depending on your microwave the pasta may need one more two-minute trip to finish cooking. Between the pasta absorbing the water and it overflowing the mug you shouldn't have more than a few drops left in the mug. Stir in a quarter cup of milk and a half cup of shredded cheese and stir thoroughly. You've achieved a tastier, less-processed microwave mac and cheese that is also cheaper than the packaged cups.