Adding a physbone collider is a great way to add realism to your character's movements in Unity3D. Here's how to do it:
1. In the Assets window, find the Model you want to add the physbone collider to and select it.
2. In the Inspector window, click on Add Component > Physics > Capsule Collider or Box Collider.
3. Configure the new collider according to your needs (size, offset, etc.). Make sure that Is Trigger is unchecked!
4. Expand Collision Matrix and ensure that everything is set up correctly for your game (you may need to consulting with your game designer or programmer). For example, if you're making a First Person Shooter, you'll probably want bullets to collide with everything except for other characters' physbones - so make sure only Player is checked under Bullet…everything else should be unchecked.
5. Test your work in Play Mode!