In chat type "/run print(GetCVar('gxApi'))" (without quotes)
It'll say 1 of 4 values:
OpenGL
D3D9
D3D11
GLL
We only support D3D9 at the moment.
Edit; just saw your other post.
Since you're in DX9, there's only a few other reasons why it'd be failing.
1) WoW is in full-screen. (Some people have said it works fine, but we suggest you run windowed mode)
2) WoW is minimized (We simply don't support this, as windows event messaging is 'quirky' and isn't real reliable on minimized windows)
3) You're not running WoW/HB as administrator (if this applies to you)