In your mocap animation, Make sure your active action is set to the mocap action, either by selecting it in the dope sheet - action editor mode, or in the NLA editor, select the strip and hit tab to turn it green.

Select the hip bone. Hide and lock all but the z-location channel. Select all channels (a key) which should only select the z-location and "grab" (by pressing the g key) the channels in the positive "y" aka up direction (constrain to up-down motion by pressing y after g) and adjust its z direction for all key-frames to one that lines up with the ground plane.

In your mocap animation, Make sure your active action is set to the mocap action, either by selecting it in the dope sheet - action editor mode, or in the NLA editor, select the strip and hit tab to turn it green (this is known as tweak mode).

Change into the graph editor, and select the hip bone. Hide and lock all but the z-location channel. Select all channels (a key) which should only select the z-location and "grab" (by pressing the g key) the channels in the positive "y" aka up direction (constrain to up-down motion by pressing y after g) and adjust its z direction for all key-frames to one that lines up with the ground plane.

Keep in mind I usually work with the dope sheet when working with 1 action because if you edit the animation, it may not always update the NLA strip. I think some of the interface inconsistencies in blender show here and are actively being worked on (eg. the tweak mode which is new in blender 2.74ish)