I currently have an event entity, which is fieldable. My question is, how do I make it appear under admin/structure, and how can I access the Manage Fields page from the admin menu? Is there a tutorial on this? Any help is highly appreciated since I could not find anything related.
You'll need to provide a menu item and page that lists information about your entity type (typically settings). If you provide multiple bundles you'll need to provide a page for each bundle.
After you've done that you can add a simple snippet to your hook_entity_info similar to the foreach loop at the bottom of node_entity_info(). Specifying the 'admin' info under your bundles will provide the "Manage Fields" and "Manage Display" tabs for the path specified. Note that if you do not have multiple bundles, the only two required keys under the 'admin' section are 'path' and 'access arguments'
Keep in mind that the path referenced in the ['bundles'][BUNDLE_NAME]['admin']['path'] section has to exist and be defined elsewhere in your hook_menu. Field module will only take care of the two tabs and the pages they link to.