Hi!
So is this in relation to Community Events?
As I don’t know what modal plugin you are referring to our quite how it works I’m making a few educated guesses here – but I think basically you would need to find a way to identify the relevant “Add Event” link in your nav menu, trap any click events and trigger the display of the modal dialog.
Ultimately I think this relates more to the modal plugin than it does to our Community Events plugin (since the same basic process would really apply whether the menu item relates to the submission form or any other page) and so it may be best to reach out to the author of this particular plugin for further advice on this one.
Does that help?