I am not quite sure what may be causing the reported problems. The only modification that has to be done to the RadFileExplorer's FileSelector demo is that you need to pass the correct ClientID of the textbox because the grid is a naming container. To ensure that you are using the correct ID you can assign handler to the RadTextBox OnLoad Client-Side event and pass its ID to a global variable.
For your convenience I have prepated a sample page using FileSelector demo in RadGrid's EditItemTemplate, please find it attached.
All the best,
the Telerik team