Noob question I'm sure, but I've purchased some items in the central marketplace and they never showed up. Where do I get them? Just says order placed and took my silver. 

It places the order and holds the silver until someone has fulfilled the item

On the cm u have 2 or when u stand before the cm 3 tabs. The second tab where u sell items has another site where ur preordered items are. I can cancel the pre-order or when u managed to bought it u get it there

Depends. If it was an item already available because someone was selling it then it is in the central market wallet. If no one was selling the item then it is preordered and the money was taken out to hold the order until someone fulfill it or you cancel it. If you cancel the order you will get the money back