When you browse to the main URL address a DM instance, a scrollable menu of public projects will appear in a window in the center of your workspace. If you are an editor on this DM server and have an account, and are logged in, you will also see private projects that you are a collaborator on. Scroll the menu and click to select the project you wish to open.

You can also browse directly to a specific project on a DM server if you know that project's web URL address. Inside a specific project, you can navigate back to the server's list of available projects (see above) via the "Return to Project List" icon in the top right of the interface: