I have been asked to reflect an object about the plane z = 1.

I have attempted this with the following code.

D3DXMatrixReflect (&m_matWorld, D3DXPlane(0.0,0.0,1.0f) );

but the D3DXPlane identifier isn't found. First of all I would like to know if this is what I need and if it is then where should be defined?

If im totally wrong here let me know. If I find a solution to this before an answer is given. I will post what I did to prevent others from getting this problem.