For MDT6 (and worked for me in MDT4) Since mirroring an assembly is not permitted in Mechanical Desktop, here's a little quick and easy workaround that usually fits the bill. Highlight all items in the assembly (this may also include any AutoCAD objects) so that the grips are visible. Make any one grip hot by clicking on the grip. Now right-click, and select 'Rotate' from the context menu. Enter 'C' (for COPY) at the command line, then 'B' (for Base Point). Now select a point on screen that lies on the line about which you want to mirror your assembly. Enter '180' for the number of degrees and hit 'Enter' again.
What happened for me is that copies (or instances) of the parts where made but, for 180 degrees they mirrored in the "wrong" plane. By using 0 (or 360) degrees or just Ortho On and physically swinging them around, you can drop them back onto themselves then use M (for MOVE) to just move them far enough apart from the originals to produce the mentally intended mirror effect.
Cheers
Aussie John