It is possible to run multiple versions of AutoCAD on your computer. You can configure Design Master HVAC, Electrical, and Plumbing to run in all of the versions of AutoCAD you have installed.
Make sure that you install the versions of AutoCAD in order of release. If you want AutoCAD 2006, 2008, and 2010 installed, make sure you install AutoCAD 2006 first, then AutoCAD 2008, then AutoCAD 2010. If you do not follow this order, AutoCAD will get confused and the installs will corrupt each other.
Design Master will need to be installed on each version in the same way. Visit the Design Master installation page for a single-user installation or multiple-user installation and follow the instructions under Configure AutoCAD.