The mouse (paintbrush) works fine for me to draw (hold left mouse button) or erase (right mouse button). I'm on Raspbian 9.6 with a Pi 3B+, The Powder Toy version 93.3. Built via scons --no-sse --release -j4. I have full OpenGL enabled although it's running at a disappointing 15 fps.
Can we get more details about your setup?