When you add a reference, they should be under COM > Type Libraries.  I think the actual DLLs are located under C:\Program Files(x86)\Common Files\Sage\Common Components.  If you take the class on SageU, they give you documentation and examples.  Neither of the C# examples require the addition of these DLLs.  Here is a thread with an example that also includes a dispatch object wrapper class to make things easier.

sagecity.na.sage.com/.../41828.aspx