Hello,
I think I got it. It's not straight forward, but for me it works.
I installed the following plugin: TT Subscribe2 Frontend. This plugin provides a front-end form for the users to manage their subscriptions. In the main php file, there are 3 places indicated by author comments where you can insert custom post types and custom taxonomies. I played a little with these and it worked. The custom post type is "ai1ec_event", and the custom taxonomy is "events_categories", both found in the database.
Please give it a try and tell me if it works. I'm not an experienced programmer, that's why I bought this small plugin for it's main purpose, but it looks like it helped for other issues too.