To make your AutoCAD .NET add-on run on multiple versions, you should target the lowest version of AutoCAD you want to support and reference the corresponding acdbmgd.dll and acmgd.dll from that version (e.g., AutoCAD 2010 if you want 2010, 2011, and 2012 support). You can get these DLLs from a machine that has that AutoCAD version installed or from its installation folder. Then, compile your add-on against these DLLs, which ensures compatibility with newer versions as long as you avoid using features introduced in later versions.