1) Make sure you have deployed your most recent version of the project to Eclipse (I think F6 might be the shortcut key from the modeler?)
2) Start the project from Eclipse as you have done
3) Open the site in your browser as is configured in the modeler (localhost: 8080 e.g.)
4) Add breakpoints as you like in Eclipse. Trigger them by performing the corresponding actions on your site.
5) Optionally, you can connect the Modeler to the instance running by going to the Debugging tab and hitting the Connect option, 'Connect a running application such as Eclipse..blah blah' (i'm paraphrasing).
You can try accessing it at: localhost:8080 (paste in browser)
But it depends on your run time port which is configured in the mendix modeler 1. Project settings - 2.Configurations (select your config name) 3.Navigate to the server tab page and here you will see the run time port 4. Append the port number to localhost: