Thesaurus in Pages 6.0.5

There is no thesaurus function in Pages v6.0.5. As Tom mentioned you must use the external Dictionary application. I keep mine in the Dock.

The Dictionary application has a Preferences panel where you can check multiple dictionaries and thesauri that are listed there. Drag these checked items into the order that you want to see them from left to right in the Dictionary application's toolbar. This will also be the lookup search order too, so if you have a German Thesaurus preceding the Oxford English Writer's Thesaurus, be prepared to expect a German result.

There are a three shortcuts for word lookup in Pages — both only requiring that the mouse pointer hovers over the word:

• control + command + D
• Three-finger tap (when configured in the Trackpad System Preferences).
• Right-click/control-click/two-finger tap (when configured)

  From the pop-up contextual menu, select the Lookup <word> menu item

I believe it's been merged with the dictionary into a single "Look Up" function. Also that it's a system-wide function, not restricted to Pages.

I can't remember what kind of setup process I used for this, but when I select a word to display the contextual menu (in my case by two-finger-tapping), I get a little pane with three sections: English, English Thesaurus, and German.

Have you gone to Applications > Dictionary.app > Preferences and made sure the box for that item is checked?