Dictionary/Thesaurus using/instructions, replace words

Sorry if this is elementary, but I would really like some clarification.

How exactly, when using the new Dictionary/Thesaurus in ML, do you replace a word in your i.e., Mail message, with a word that shows up in the Thesaurus?

I can get the little box to show up and I can get the Dictionary window to show up. I can click on the new word I want, but I still can't figure how to make that word replace the word in my Mail message.

Example:: I'm typing a message, I type the word, "replacement." I highlight it and go through the clicking to get the Dictionary/Thesaurus window to appear. I see the word, "substitute." I want to use that word in my Mail message. I click on it and it opens to that word with its definition and thesaurus choices. The word, "substitute" is now in the search box.

What are the way(s) to get it into my Mail message? Is the only way to highlight it and copy and paste?

Thanks for some clarification. Apple's Help doesn't get that far.