This is not directly possible in WriteMapper, but you can still do so with a few extra steps.
As a workaround for getting the visual of the mind map to print, you can take a screenshot of the WriteMap you're working on on your computer, which will get saved as an image file. You can drag the edges of the WriteMapper window to resize it, as well as use the zoom controls of the app, to ensure that your WriteMap fits into your screen well.
So, you'll now have an image of the mind map. From there, you can also edit the image using your graphics editor of choice, and print it from your computer, if necessary.