Leanne,
RTF already has spell check.
You need to get set up with a WYIWYG editor by doing the following:
Click "user CP"
Then select "edit options"
Then scroll to the very bottom where it says "Miscellaneous options"
In the field where it says "message editor interface" click the dropdown and select "Enhanced Interface - Full WYSIWYG editing"
Once you have a WYSIWYG editor, you will see all the smilies, the different fonts, links for images, quotes, etc. In the top right corner you will find a checkmark with "ABC" above it. This is your spellcheck feature, right here for use on RTF. It requires a download, but it is available if you want to try it out.
Chris