Your assemblies can be anywhere you like. All you need to do is add the path to them in the "References" text box in the Macro Editor, and make sure your assemblies are compiled with the same version of the .NET framework (4.6).

I've attached a screenshot showing a Macro that I use with an external assembly.

I hope this helps!

The way I usually test it is to add the DLL to the references and compile to see if it throws an error. The error usually looks something like this if the assembly is using the wrong .NET framework:

"an attempt was made to load an assembly from a network location which would have caused the assembly to be sandboxed"

You can try a different version of the DLL, or you can right click on the file itself, click "Properties" and unblock the file. More details here:

https://stackoverflow.com/questions/15238714/net-local-assembly-load-failed-with-cas-policy