What sort of DLL are you talking about here? A .net DLL can just be dragged in to unity (assuming it doesn't have any weird requirements like System.Windows). If it's a native dll (from C++ or something), you can read about it here: http://docs.unity3d.com/Documentation/Manual/Plugins.html
If you want to use Unity assets from the plugin, you'll have to build an interface from Unity yourself. Read the info I linked for more information.
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