Hi! I believe the following should solve all your questions:
Red vertices just show the borders of your mesh, so you can tell where a hole is. You can turn this off in Viewport Display / Non-manifold Outline.
Symmetry is on the left either because your model is rotated or its orientation is backwards.
More on that here: https://cgcookie.com/lessons/working-with-symmetry
For the install issue, try using RetopoFlow 3.2.9 and let me know if that works! I’ll update the documentation if it’s mistaken.
