Hello Jagat,

Thank you for writing.

The assembly that needs to be referenced is Telrik.WinControls.UI.dll, and the namespace of RadMessageBox is Telerik.WinControls. Please refer to this help article, which demonstrates how to make use of this compoenent.

I hope the provided information addresses your question. Should you have any other questions, do not hesitate to contact us.
 
All the best,
Stefan
the Telerik team