As with any other import into MB, the trick is to have a suitable rig/character for the BVH data to drop onto. This is normally achieved by loading up the BVH file and picking a T-Pose frame if it has one. From there, you manually build the required bits for the character prior to characterising.
At that point, you can then drive any other character in MB to plot the data t oallow export to your preferred 3D package.
One thing I have noted, many times, BVH data can differ in it’s structure and the number of bones / bone names can vary so you may have to build more than one rig to suit.
If it’s any help at all, attached is a ZIP file with the various BVH rigs I use in MB depending upon where I’ve picked that data from.
…Scratch that, the pathetic 1MB limit for attachments on this board means I can’t upload the full thing here.
Best I can do is provide a file with one or two examples, sorry 'bout that, needs must, you can contact via PM and I’ll pass on my MSN / ICQ details if you need other examples.