To add a persona to your impact map, first you need to click on +ADD TILE under PERSONAS title:
Second, you can either choose among personas that already exist in your project, or create a new persona by clicking on the UPLOAD IMAGE button.
Note that there is a limit to 1 persona on the free plan. However, you can have multiple personas inside of an impact map (since there is not much value in impact mapping with only one persona). But you won't be able to edit personas that exceed the limit inside of the Personas tool.
Once you imported some personas into your impact map, you can choose between them to see or type in additional info like goals, pains and so on. To select a persona you can click on a sandwich button in the upper-left corner of that persona tile.
Also, if you grab this icon you'll be able to drag and drop this persona and the whole branch of tiles it contains.