Q: When using Zap A Card in Fluxx, can I take cards out of the discard pile?

A: No. The card says you can take any card “in play” on the table. That includes: the current Goal, any current Rule (not the Basic Rules, of course), or any Keeper or Creeper in front of any player. Note, of course, that Creepers cannot be held in your hand, so they go back into play in front of you if you steal them from someone else, instead of going into your hand.