To get started using the Invision Marketplace API, just head on over to your Account Settings -> My Marketplace Settings and create your client using the form provided.
You will also need to apply for the Clients member group using the Applications link at the top, and selecting the application for Invision Power Services, Inc. Client, and clicking on the Apply button. Once verified, you will be moved to the Clients group, and will then be able to download, and upload to the Marketplace.