Below is a list of tutorials about writing add-ins for Enterprise Architect. Best to read them in the proposed order.
The first baby steps in creating your own add-in
Making your life easier with the add-in template
You’ve created something but now you need to see if it works (or why it doesn’t)
How to make others happy with the fruits of your labor
Getting serious about add-ins? Using this frameworks saves a lot of work and enables re-use of your hard work.