Visio 2016 : Can't access UML diagram properties

In the UML shape design for Visio 2016, the Class shape is a Visio "list" and each text shape -- what you want to be the attributes and operations -- is a "member" of the list. That's why when you drag a new Class onto the page it looks like the one on the left To turn it into something like the one on the right, there are four things to know:
1) To turn a generic member into an attribute or operation: select a member shape and start typing (or you can double-click and then type).
2) To add a new member to a class, drag a member shape from the UML Class stencil. If you hesitate when the cursor is over the Class shape, you'll see an orange insertion bar (see image below), which will help you position the separator exactly where you want it.     or
     Right-click any existing member and select either Insert 'Member' Before or Insert 'Member' After.
3) There is a separator shape (the dotted line) in the Class object. If it gets deleted, or you need another one, drag a Separator shape from the UML Class stencil and drop it between any two members.  
4) You can change the sequence of members in the Class shape simply by selecting them and dragging them up or down. Pausing slightly while doing this will cause the orange insertion bar to appear to help you position the shape where you want it, just like adding a new member or separator.
 
BTW, if you want more background info on containers and lists in Visio, I wrote a set of three articles for EE a couple of years ago. The articles probably contain more information than you want at the moment, but containers and lists are pretty cool structures that you can use for a variety of purposes. You'll find them used for various purposes by Visio itself, e.g., swimlane diagrams, wireframes, data graphic legends, but you can also use them yourself if you want help organizing part of your diagrams. The first article is here and it contains links to the second and third.