see this:
http://abscnc.sf.net/haydn/hva_editor_guide.jpg

Remember that an HVA is basically an animation; it comes in a series of frames (possibly just one). You can create a new HVA by selecting 'New' on the file menu. You must have the vxl made first.
At first, all the sections will be piled in a heap (well,.... that's what it looks like). Select the section and move it.
The View menu is pretty straight forward. View 'Animation browser' is very important. You can add new frames of animation by using the buttons or under Edit, 'Frames'.

Good luck, it isn't that complex really.