RadRichTextBoxRibbonUI is the ribbon control which is wired with the commands of the editor in most examples. It is not necessary to use it, you can use the editor without any UI or implement your own toolbar/context menu/etc.
However, judging from your post about the StylesGallery, you seem to be using the UI that comes with RadRichTextBox out of the box. In this case, you must add the assembly references to the dlls that contain the functionality.
The assembly that keeps the implementation of RadRichTextBoxRibbonUI, the styles and lists gallery, etc. is:
This assembly has a dependency on the assemblies listed below, which also have to be referenced in your project:
You should make sure that these references are added to your project and that they are of the same version.
The error from your post that "Telerik.Windows.Documents, Version=2012.1.326.40" cannot be loaded means that one of the assemblies that have a dependency on Telerik.Windows.Documents is Version=2012.1.326.40. This could be the RichTextBoxUI or one of the other assemblies that the RichTextBoxUI depends on.
On a side note, I tested the 2011.2 716 version of the controls, as well as 2012.2 725 (the Q2 SP1 release) and everything worked as expected. Please, verify that the assembly references are correct and open a support ticket with your project, so that we could assist you further if needed.
All the best,
the Telerik team