Where is berry crusher in emerald?

The main problem is that you MUST have a wireless adapter or cable connected to your GBA. Without it, it is simply inaccessible.

You must also have a Powder Jar that can be obtained at the marketplace in Slateport City.

With these two requirements met, go inside any Pokemon Center and go up the escalator. Talk to the servicewoman on the far right (Direct Room). After your game is saved, you will be asked to go in the Trade center or Colosseum.