Yeah this dialog is unnecessarily complex.
Try this:
On the right hand column called "Build or edit this menu", double click on "Tools" and then on "Spell Checker". This will show your three current spelling menu items.
On the left column called "Application commands and menus" the pull down menu should have more options available, change to "All commands by theme".
Expand "14 Add-Ons" and then "Spell Check" and then "Spell Checker".
The additional menu items should be available for you to add.
Note the settings at the top of both columns on my previous image.
Hope that works. (They could have made it a little easier. I spent way too much time figuring this one out.)