Motion Mixer. This lets you overlay different tracks of animation and blend them together. You create a mix of all the tracks and then apply that to your biped. You might need to first load the mocap onto a biped and then save out a .bip file from the biped menu. I'm not sure what other formats Motion Mixer imports besides…
While in mixer mode (little egg beater in biped menu) there are no curves or keys to edit, at least none that relate to what you see in the motion mixer. You'll have to create a mixdown and apply that to the biped, then exit mixer mode before you can edit the keys and curves on mixer animation. There could be keys already…
The max viewport gizmo is kind of weird. There is a drop down in the main toolbar that allows you set the viewport pivot to local space. What this allows you to do, is see on the bone in the viewport which axis is doing what. If its set to anything else but local and you rotate it on X it might move on Y and Z in the curve…