Some of your difficulty accessing the Media Library may be resolved by my reply to your other thread, but I can provide a bit more detail about these sections. We would like to update our documentation to include these new sections at some point, but for now, hopefully, this will suffice!
The Media Library is accessed through its own section in the menu. (As mentioned in your other thread, this is created by a new schema file). When the Media Library is enabled in the site settings, upload fields will now have the option to "select from media library" - which will display items previously uploaded to the Media Library which can then be added to the field. The Media Library menu item can have its access controlled like a regular section when editing a user account. The upload field functionality is always available to all users, regardless of access to the menu item. So while it's possible for some users to be restricted from whether or not they can add/delete items from the Media Library, when selecting items to add to an upload field all users can see all items in the library.
The Audit Log can be found as a sub-item under the "Security Settings" Admin menu item. You'll have to visit the Security Settings page for the menu item to be visible. This section is also controlled by a schema file (_log_audit.ini.php) so if the menu item is missing, you can first double-check that the schema file has been added. Only admin users are able to access the audit log.
Let me know if you have any other questions!