It may be different in the Create A Pattern program, but from within the game, you have to "Share" your items (Sims, lots, furniture, etc.) to make them available to the Exchange. This is done from Create A Sim or Build/Buy mode. Then from the Sims 3 Launcher, click the Upload button, then check the boxes of the items you want to upload, then click Upload. We also have our own
Swap Shop where you can share items you've made. It's a different process to upload, but not really any more complicated.
Have you read the
Create A Pattern Guide? It may have some information that will help you.
Sorry, Audren posted while I was still typing.