Switch to the Financing module (1) and click "+" (2).
Choose a name (3) and a linked budget for this plan (4), that will be used for project settings.
Decide if you want to start from scratch or use a financing plan template file (*.apmst) (5)
(You can save an existing financing plan as a template in MENU->PROJECT->SAVE FINANCING PLAN AS TEMPLATE)
And click Ok. (6)
If you decided to start with an empty plan, start by creating Categories, Groups and Financing Sources (1).
Groups have to be inside a category. Sources have to be inside a group.