The Manage Schemas page is accessed like any other page in the Console; you simply click the appropriate link in the navigation pane:
That’s all there is to it, although there are couple of things to keep in mind here. First, the Manage Schemas link is only visible if you hold a Console role that gives you access to schema information: if you don't have such a role you won’t even see the link, let alone be able to access the page. In addition, clicking Manage Schemas only toggles the display of the individual entity types used in your organization. To navigate to a schema page, click the name of the entity type itself (e.g., user).