Geert Bellekens did a nice tutorial on how to build addin’s for Sparx Systems Enterprise Architect:
Create your first C# Enterprise Architect addin in 10 minutes.

The post also contains this nice diagram showing what it takes:

Here at LieberLieber we do it a little bit different using our own framework in between – and we don’t install into GAC. Nevertheless you should get the point how to get started. If you don’t want to do it yourself feel free to contact us Winking smile 

In our opinion it is vital for an addin project to be successful to offer an MSI setup for deployment.