Firstly, you should do some usability testing with your interface to see how well users pick up the functionality. Highly recommended and effective.
However to give some advice regarding your interface:
You should provide a more obvious way to enter edit mode. Users are unlikely to recognise invisible or hidden features. They will not likely guess that clicking on an item will edit it. Also they should be able to delete it without going into 'edit mode'.
A great way to overcome these issues is to show an edit and delete button next to each item on hover. You can keep the 'on click' edit mode but provide more obvious access to your users.
You should add a functional 'save' button to your edit and add modes and keep them consistent. Users will not know for sure that hitting enter will save their content, having a save button will give them security. Some users are not familiar with hitting enter and will instinctively look for a save/add/confirm button.
Keep the interfaces the same - hitting 'add new item' in the first instance creates an item, thereon after the user is editing this item. To illustrate - when you 'create new folder' in windows explorer you don't need to hit another 'add' to add the folder. If you exit the name definition field without changing the name the folder still exists as it's been added.