ClipMate uses a spellchecker called Addict3 by Addictive Software. There are numerous dictionaries that you can download from their website. They have dozens of dictionaries, including many languages, and specialized dictionaries with technical terms or even one that specializes in auto-correcting American English to British English.
These dictionaries are in ZIP format. Download from the Addictive Software site and open with an Unzip program (such as WinZip from www.winzip.com) and place the resulting files in your ClipMate program directory (such as c:\program files\clipmate6).
Then restart ClipMate, and go into the spelling configuration. You will have a new main dictionary which you can activate by turning on the check-mark. If you add any of the "auto-correct" dictionaries, then add them under "custom dictionaries". The Thesaurus does not need any configuration - just place either the American or British dictionaries into the directory mentioned above, and it should be active. Right-click on any word, while in ClipMate Explorer, and you can access the Thesaurus.