How do you reflect and object on a plane?

[OReubens]

it's D3DXPLANE (all capitals) unless you have some specific library/class in use.
The source plane needs to be a pointer.

Assuming you have the C++ extensions enabled in DX:

D3DXPLANE z1(0.0, 0.0, 1.0, 0.0); 
D3DXMatrixReflect (&m_matWorld, &z1 );

Whether this is what you're after is another matter.