Jazzcnc is giving a correct solution. You can use planes or construction geometry to align parts in order to mate. Create reference geometry to either create planes or use the geometry directly to orientate your parts.

I am assuming this is an assembly you are trying to mate and not an in-context body? If stuck come back and I will show you how.

Hi all by the way, new member and CSWE for reference. ;) Always glad to help with anything SW related.