The Easycom for .NET assembly is located into the installation directory:
C:\Program Files (x86)\Easycom .NET\ System.Data.EasycomClient.dll
Into the project properties, right click on References and add a reference:
Browse directories to indicate this path:
C:\Program Files (x86)\Easycom .NET\ System.Data.EasycomClient.dll
Then add the reference:
Now, into the Program, you can add the directive:
using Easycom;