Hi Grace Pahuasi,
Thank you for writing. There is no way to include an editable control in the standard ToolTip. That is because - by specification - the ToolTip serves only for displaying text. You have to keep in mind that the ToolStrip hides when you move the mouse outside the object that owns it. This makes editing impossible. Instead of adding a RadTextBox in the way that you have tried in your code, I recommend you create a custom form object with a RadTextBox inside it that you can show and hide according to your needs.
Do not hesitate to contact me again if you have any other questions.
the Telerik team