"Using Mono DLLs in a Unity Project"
The path to the Unity DLLs would typically be:
An example of building a DLL is as follows:
Mcs-r:/applications/unity/unity.app/contents/frameworks/managed/unityengine.dll-target:library ClassesForDLL.cs
Once compiled, the DLL file can simply be dragged to the Unity project like any other asset. The DLL asset has a foldout triangle which can being used to reveal the separate classes the library. Classes that derive from Monobehaviour can is dragged onto Game Objects like ordinary scripts. Non-monobehaviour classes can be used directly from other scripts in the usual.
Create Dll:in MonoDevelop, you do this by selecting File > New > Solution and then choosing C # > Library.
Reference Dll:in MonoDevelop, you should open the contextual menu for Referencesin the solution Browser and choose Ed It References. Then, choose the option . Net Assembly tab > File System > select File.
Reference: http://docs.unity3d.com/Manual/UsingDLL.html
Using Mono DLLs in a Unity Project