Download a trial by clicking on this link
The download is a zip file containing "CalcEnginePlus.dll" and the "CalcEnginePlus.xml" intellisense file.
The trial never expires, so you can take as long as you need to evaluate it.
Unzip the file and add a reference to "CalcEnginePlus.dll" in your .NET project.
Instantiate a "new Parser()" and off you go! Have a look at the documentation and code samples