In the Solution Explorer right click "References" and "AddReference":
Add reference to SautinSoft.HtmlToRtf.dll (The Library is located in the subfolders of the folder "Bin" of the Package "SautinSoft HtmlToRtf .Net Trial (N.N.N.N)"
For example: Bin\Net 2.0\SautinSoft.HtmlToRtf.dll or Bin\Net 4.0\SautinSoft.HtmlToRtf.dll.
Well done! Now your project is able to convert HTML to RTF format and merge RTF documents!