Spellcheck is no longer a default tool in the editor toolbox. There are several reasons for this: it was redundant (most browsers have native spell check), it used valuable resources (causing pages to load slower), and it was rarely used (save by a small, but loyal, following.)
Here are some browser specific tips to up your proofreading game:
Safari users just need to activate spell check from the menu bar: Edit > Spelling and Grammar > Check Spelling while typing
Internet Explorer uses an "Add on" to handle spellchecking: here's a short video that explains how to turn it on.
In any case, this should activate a spellchecker within your browser that will be available on Finalsite (and any other website you visit.)