You can download it and install it from here. I believe that is a direct link to the spell check and this is a link to the page.
Then to spell-check in a window right click up at the top (not where you type) and go down the list until you see spell check. It's amazing!
First, after you finish composing a reply message, right click your mouse anywhere on the white area above (beside the Subject line will do nicely) and you will see something like this:
Then from the drop down menu, select "Check Spelling" (see above). You should see
something like this now:
Thanks to Stubby from "The Fly Trap."
