As Iva mentioned, in case you have a hotfix version of the controls, you'd have to download a zip, containing the demos. This
is a link directly to your profile, where you can find the files.
Once you've downloaded and extracted the archive, you can find the Tokenizer class here:
In case you have included the examples when installing the controls, the tokenizer should be at:
C:\Program Files (x86)\Telerik\RadControls for Silverlight Q2 2012\Examples\RichTextBox\Parsers\Tokenizer.cs
I hope this answers your question! Get back to us if you need further assistance.
the Telerik team