To insert an eccentric transition or an eccentric reducer using Design Master HVAC, you need to change the Horizontal Alignment on your ductwork to Flat on Right or Flat on Left.
To do this, use the Query Duct or Edit Multiple Ducts command and change the setting there.
Ribbon: DM HVAC->Ducts-> Query Duct / Edit Multiple Ducts
Pulldown Menu: DM HVAC->Duct Centerlines->Query Duct / Edit Multiple Ducts
You can also set this when initially inserting the duct.
When you change this, the duct centerline will define the edge of the duct instead.
If you change the offset in the middle of the run, the ductwork will not line up. An entire run of ductwork should be set to the same offset. You can change your offset when you branch at a tee or when you turn at an elbow.