The thesaurus icon 
(which can be selected from Edit in the menu bar) is hidden because no thesaurus is currently selected. Currently only US and British thesauruses are available. If you would like to use either of those, go to the Menu, click on Tools -> Options and, on the first tabsheet, click on the drop-down list to the right of 'Thesaurus'.
If the only selection is 'None', it means that neither of the thesauruses will have been downloaded. For information on downloading them, go to the Dictionary & Thesaurus List item.
If there are choices, please select the one you require.
Once a thesaurus has been selected, the thesaurus icon will be visible.