Navigation between table cells using the tab has not been implemented in RadRichTextBox yet. Currently, pressing the Tab key changes the paragraph indent or inserts a tab. You can also set the AcceptsTab property to False in order to prevent this behavior and have the editor pass the focus to the next control on the page.
However, there is a logged request for implementing the caret walk using the arrow keys. Here is a PITS issue, which you can use to track our progress:
PITS Issue: http://www.telerik.com/support/pits.aspx#/public/silverlight/6587
the Telerik team