Specify a Y decenter of -150 mm in the surface #2 property window.



From the manufacturer's specifications, the off-axis distance is 150 mm and the physical diameter of the mirror is 203 mm. Specify the correct aperture size and location.





Open the 3D layout



Note that the rays are moving away from the coordinate system. In order to center the image surface and make it orthogonal to the chief ray, insert a coordinate break surface before the  image surface and place a chief-ray solve on the Y decenter  and the X-tilt parameters. ZEMAX will automatically calculate the amount of decenter and tilt needed to make the chief ray hit at the center of this surface at normal incidence.



Update the layout.



Perfect!