ID3DXFont::DrawText

INT DrawText(
  [in]  LPD3DXSPRITE pSprite,
  [in]  LPCTSTR pString,
  [in]  INT Count,
  [in]  LPRECT pRect,
  [in]  DWORD Format,
  [in]  D3DCOLOR Color
);

http://msdn.microsoft.com/en-us/library/windows/desktop/bb173962(v=vs.85).aspx

Note: Remember to use a non-zero alpha component for the Color parameter (so, invisible text is not the droid you're looking for...).

C