Hi trevbennett, and welcome to Tikiwiki community!
It did work for me weeks ago (when I made the screenshots in documentation: http://doc.tikiwiki.org/Wysiwyg+editor ). This might mean that it was temporarily unavailable at cvs or so (there is an issue opened against 1.10cvs upgrade that at dev.tw.o...
I do not know what's wrong; anyway, remember 1.10 is not stable yet...
Use 1.10 only at your own risk; it's not avdised for production sites yet, unless you have a (probably payed) coder with you to solve the issues that might arise. I'm not a coder, though...
However, if you browse cvs http://dev.tikiwiki.org/CvsTips" class="wiki wikinew text-danger tips">http://dev.tikiwiki.org/CvsTips, for instance, through here, you will see that setup_fckeditor.php and is there in place (and there hasn't been changes for 4 weeks).
And if you download the tarball suggested at http://doc.tikiwiki.org/Upgrade+1.9.7+to+1.10
you find the file also there inside.
By the way, if you can help keeping the "upgrade to 1.10" page updated, please help the community doing so, if you are sure that there is something missing: