You add a new topic to a map with the following steps:
In the view DITA Maps Manager click on the action Insert Topic Reference that is available on the toolbar and on the contextual menu. The action is available both on the submenu Append Child when you want to insert a topic reference in a map as a child of the current topic reference and on the submenu Insert After when you want to insert it as a sibling of the current topic reference. The toolbar action is the same as the action from the submenu Insert After.
Select a topic file in the file system dialog called Insert Topic Reference.
Press the Insert button or the Insert and close button in the dialog. A reference to the selected topic is added to the current map in the DITA Maps Manager view. The button Insert and Close closes the dialog.
If you clicked the Insert button you can continue inserting new topic references using the Insert button repeatedly in same file system dialog or you can close the dialog using the Close button.