The original poster was looking for functionality to allow visitors to add markers to his existing map on the front end and not the back end.
However, should anyone wish to allow your editors or subscribers to allow access to the plugin in the backend, please follow the following instructions:
Open the basic plugin in an editor and search for "manage_options". Change it to the following depending on what user role you would like to be able to create, edit and add markers to maps:
Editor: change all existing "manage_options" to "edit_pages"
Author: change all existing "manage_otpions" to "publish_posts"
Contributor: change all existing "manage_options" to "edit_posts"
Subscriber: change all existing "manage_options" to "read"
There should be a total of 3 "manage_options" to change.