please check out the documentations on the lidgren page on how to use it. there is no mechanism on the unity side to just hook it in, you have to develop the messaging, message handling, object syncronization, UUID generation etc
thats exactly what you need to do But lidgren features means messaging capabilitys. all the handling in relation to it, sending what when and where and interpreting that on the other end must be self implemented
drop it in and then import the namespace / using the namespace (depending on the language) what classes etc it offers and how it works for communication and what it requires is mentioned on the lidgren page