I believe you need to add Attributes in the UML nomenclature. Double click
on the Class shape on the page to see it's properties like Attributes.
You can learn a bit more about Visio UML diagrams on Office Online :
http://office.microsoft.com/en-us/visio/CH100648901033.aspx
Note that these help topics assume you already know UML notation, and are
looking at how Visio uses it.
By the way - Visio implements the 1.2 UML notation version, which is
relatively old.
Barb Way
Product Support - Visio
Microsoft Corporation
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