The Catalog tab lists all of the layers that the current user has access to are available to be displayed on the map. Check the box next to each of the layers that you want to be displayed; it will be added to the Active Tab.

Layers are grouped under headings. You can toggle all layers within a group to be active/inactive by checking/unchecking the box next to Group's name.

Click the Add button to add an external layer. See Adding External Layers.

You can filter the layers listed on the Catalog tab by using the Search layers textbox next to the Add button. As you enter characters, layers that contain the values entered appear on the Catalog tab. Select the appropriate layer by ticking the box next to the layer name.