I love this plugin, I use it every single day, but it needs to be updated to point to thesaurus.com rather than thesaurus.reference.com. There's a quicker fix, however, than searching for a file on Google.
Go to Preferences, then Search and find your search engine list. Highlight 'Thesaurus - Reference.com' and click on 'export to file...' Save the file to your desktop and open it with a simple text editor. Change all references to 'thesaurus.reference.com' to 'thesaurus.com" and then import that file into your search engine list in Preferences (using the 'Import Search Engines From Files..' option.
You'll end up with 2 Thesaurus search engines, so delete the old one ('Thesaurus - Reference.com') and leave the new one: Thesaurus.com.