If you highlight a word and press Shift+F7 or select "Tools | Thesaurus", you will get a list of all the similar words that you could use instead of the current word. In the instance below, the word "press" was highlighted in the editor and the alternative word "deluge" was selected to replace it.

I don't see this option!

If the Thesaurus menu option does not appear it is simply because the Thesaurus has not been installed. Please download it from our site:-

  http://www.Fast-Help.com/dictionaries.htm

and unzip it into your Fast-Help installation folder. Then please rename it to "Roget.adt". When you next load FastHelp the "Thesaurus" option will appear on the Tools menu.