If you are at a repl, this should work:
(use 'vijual)
(def foo (draw-tree ....
use refers another namespaces symbols into your current namespace so you can call them without qualifying them. require loads the namespace but would require you to reference symbols with it:
(require 'vijual)
(def foo (vijual/draw-tree ...
When I did this the function was invoked, but still had errors. In particular, you have (9/2 6 13/2)
etc - this will attempt to invoke 9/2 as a function. I don't quite understand exactly what you are shooting for (in particular with the map) but this does a start of something:
(def foo (draw-tree [[:vec [9/2] [6] [13/2]] ]))