Compiling Nav code with dotnet variables

I hope this isn't a dumb question but I can't find an answer anywhere so I will ask.
I have inherited some 2009 RTC code which references .net assemblies. These are not registered but copied to the Add-ins directories.
Server side objects are executed from ..60\Service\Addins ok but when I compile where does Nav look to make sure the dll version is correct? I have copied the same dlls to the add-ins folder under ..60\Client and ..60\Role Tailored Client but still get a compile error. What are the rules and tricks for compiling and executing .net objects like this?
Thanks!