I’ve been struggling with this for awhile so I figured I’d try to save myself some frustration and see if anyone has already cracked this nut. I’ve tried (and am still trying) to do this with both the Mocap Tools and MakeWalk with little success. I know Manuel Bastioni Lab has skeleton retargeting slated for 1.5, but there’s no guarantee that this will work with the Mixamo skeleton either, so I see no reason to just wait for it’s release.
Edit:
I got it! I finally figured out a way! Download the BVH for 3dsMax version from Mixamo, and in the Source Armature panel of MakeWalk, uncheck Auto and select the 3dsMax armature.