I've been trying to make macarons using almond meal, and they turn out very lumpy and grainy. Should I be using almond flour instead? Or doing something else to make the powder finer?
In any case almond meal and almond flour are essentially the same thing, both are almonds ground into a fine (or not so much in your case) 'flour' or 'powder'. Apparently, almond flour is made from 'blanched' (almonds which have had their skin removed) almonds whereas almond meal can be made from either almonds with their skins on (whole) or 'blanched'. I have, however, always found them to be used interchangeably.
If your problem is graininess, then you could either sieve the ground almonds before using or blitz/grind them finer in a food processor until the desired consistency is reached, however if you're grinding them in a food processor make sure you do it in small batches or if you're making macarons blitz them with the icing sugar (for the tant pour tant) to avoid making almond butter. Personally, I would do both to achieve a fine powder.
Almond flour and almond meal are pretty much interchangeable terms. What you are essentially looking for is the finest ground almonds you can get hold of.
I buy ground almonds (because they're easily obtained) and sift them to get the finest bits. This isn't very economical (90g of fine almonds from a 150g bag, usually) but you can use the coarser bits in other baking.
According to this article on how.com (regarding low-carb diets), if you use only blanched almonds, instead of ones with skins, you'll get a finer flour: