If you navigate to Content > Items for the first time, you will notice that you will see an empty list, this is because you first have to mark items as ChannelEngine (CE) items before they can be synchronized with ChannelEngine. 

To do so, go to Items (the standard BC item list), open an item, and click on Create a new CE Item.

After this, you will see a notification and a link that takes your straight to the CE item you have just created. From now on you will also find this item under Content > Items, and you can start to work on the details that define how your items will look on the marketplaces.

There is also a final release switch. This is to make sure that you can work on the item content for as long as needed. Only once released the item will be pushed to ChannelEngine.