To add a persona to your impact map, 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 3 personas on the free plan.
Once you import 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 can drag and drop this persona and the whole branch of tiles it contains.