Little plus sign in the corner that views the tree is missing

Is this what you need?

You need to be editing a feature in order for the flyout FeatureManager tree to be visible.  This is because editing a feature opens the PropertyManager tab and you can no longer see the full FeatureManager Design Tree.  You'll then see the "+" or ">" in the top left corner to expand the tree for selections.

In the screenshot, you're still in the FeatureManager Design Tree tab so you have access to the full tree and the "+" sign won't be available.  For example, click on the "Mate" tool in the CommandManager.  This opens the PropertyManager tab for mate options and then you'll see the "+" to have access to the tree.