I just updated Blender to 2.78, and now I can't move the viewport with Option-Click and drag. I can select objects but I can't move the viewport.
Does anyone know how to solve this?
It's possible that not all of the settings were copied over from your old blender version into 2.78 when you installed it.
As far as I know, the option to move the mouse with Option (or Alt)+Click and Drag isn't enabled by default, but can be enabled by checking the 'Emulate 3 Button Mouse' box under the Input
panel of the User Preferences.