Since this was posted under RichTextBox header I'm assuming the original poster meant that it wasn't working for RichTextBox controls.
I see the same behavior. If IsSpellCheckingEnabled is set to True then misspelled words are highlighted but it doesn't suggest alternatives.
I tried using
Friend WithEvents rscMain As Telerik.WinControls.UI.RadSpellChecker
where Me.rtbComments is a RadRichTextBox control and it brings up the spell checker form and suggests alternatives but it when you're done correcting the spelling and click on Ok (or fix the last misspelled word) it doesn't copy the changes back to the RichTextBox control. I had to switch to using a RadTextBox control.
I also tried a RadTextBoxControl control and got errors when trying to use the spell checker with it.
I add this to my form load event:
rscMain.RegisterControlSpellCheker(GetType(RadTextBoxControl), New TextBoxSpellChecker)
But when I tried to check the spelling I get:
Unable to cast object of type 'Telerik.WinControls.UI.RadTextBoxControl' to type 'System.Windows.Forms.TextBoxBase'.