Catnip works well in most cases. I taught my girl to use her cat tree with a wand toy - get her playing and into the toy, then slowly drag the toy around and up the tree. She should naturally pounce it, and in the process realise what a great place it is to scratch. Once she's made the connection, she should naturally chose it for scratching on
Usually I just read the forums occasionally, but I decided to join today when I saw your question, just in case by some chance you have a kitty that doesn't start using it with catnip. (Like mine. She doesn't react much to catnip.)
I had to show her it was fun to use her condo by having her chase a cat wand toy onto the different levels. It only took a couple play sessions to eventually get her onto every level, and now she loves it. I had to do the same thing to get her to try the scratching post. Basically, get her to climb up the post as she was hunting the wand.