I've been playing Minecraft for awhile, and I quit for awhile. But I decided to come back after it entered beta with all the updates and whatnot. I decided to get one of the custom painterly packs, and as far as I know, did everything correctly. However, the pack isn't showing up in the texture packs/mods management thing on the main screen.
Start the game.
Click on "Mods and Texture Packs".
Click on "Open texture pack folder".
Doing that should open up a folder. Place the zip of your texture pack in that folder. Do not extract the zip; leave it compressed.
The texture pack should now be selectable in the menu. If you don't see it, try clicking and dragging up. Note: scrolling up and down with the mousewheel does not work.
Edit: I'm assuming you're trying one of the Painterly packs that are 16x16 resolution. If it's some other resolution, you'll need to get Xau's HD patcher first, and follow the directions there.