I looked at your rig and you've made 2 very common mistakes.
1. When binding, never "Maintain Max Influences," especially if you need to paint weights.
2. You can't change weight assignments, of weight assignments are locked.
Generally, it is a bad idea to use many of the default bind settings. If you let Maya maintain the maximum number of influences assigned at bind, that means you can't reduce the number of joints influencing each vertex. In most cases, having each joint affected by 5 of the nearest joints results in a deformation mess.
Here are a couple of links to old videos that'll cover the basics and may provide further insight.
Smooth Binding Basics