In order to make sure that the property is properly called, you will need to use slightly different syntax:
| function radGridDumOnKeyPress(sender, eventArgs)
| alert("Is Ctrl pressed: " + eventArgs.get_isCtrlPressed());
the isCtrlPressed() function starts with lowercase letter.
Also, unfortunately, you cannot detect the insert/delete key presses. Explorer doesn't fire the keypress event for delete, end, escape, function keys, home, insert, pageUp/Down and tab.
I hope this information helps.
the Telerik team