You need to add the Lidgren.dll
as a reference to your project. Take a look at the picture below (Made in VS 2012, but same for any other editions)
Hit browse and find your Lidgren dll.
This is not the place for legal advice, so don't tank us in the place of a lawyer, but as you can see on the homepage, it is under a MIT license.
... including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software...