This is a issue with the tinymce editor SilverStripe uses. It overrides right click so it can provide a custom dialog box for the editor window. Not too sure how you would get around that apart from writing the content in word and using spell check in that and pasting it into the CMS.
Thanks, Willr, for the explanation. I also use WordPress quite a bit and have clients using it so I am used to this feature when creating postings and other entries. Obviously not a top priority, but it would be nice to have a spell checker available since this is a tool that is pretty widely expected as part of the suite of editing tools.
Now that I think of it WordPress uses tinymce too. They musyt have a way around this or they are not using a custom dialog box.
On my Mac a LEFT click with the Control key held down brings up the contextual menus from the Mac operating system which include "check spelling" - then misspelled word are underlined in red and a LEFT click with Control key returns a suggested correct spelling.
The pending release which is due within the month will have a new version on TinyMCE which has the spell checker enabled.
You can download the latest code (which has this new version of TinyMCE) from dailybuilds.silverstripe.com or via subversion. Note that this is unstable code so who knows whats broken but if this is a critical