Since I see html-el tags in your JSP, I'm assuming you're using some version of Struts 1. When you use indexed properties in Struts 1, your ActionForm must have a property containing a collection that matches the name of the property in the JSP. That property must also have indexed getters and setters.
What you've done is place the list in Session scope, which is fine for rendering the checkboxes, but not fine for getting a result back. The solution, then would be to place the list in the ActionForm something like this:
If you want to put the ActionForm in request scope, you will need to provide lazy initialization for your indexed getter. See this entry in our Struts 1 FAQ for more information.