Click Manage > Asset Definition Manager in the left navigational menu

Click on the Create New button at the top of the Asset Type pane or right click on an existing class or category and click Create New in the context menu

In the modal that appears, fill in the fields with either new or existing values to define the Class and Category, and Type (note, when entering brand new values, you must click the resulting underlined word to select it)

Lastly, click the Create button. A new, blank, asset definition will be created. 

From here you can begin by customizing the icon and color of the definition and creating sections, and elements within the sections.