Be careful when entering name and serial number so that you don't mix up uppercase and lowercase letters and that you don't enter additional space characters. Note that many applications do add a space character to the selected text when they copy the text via Drag & Drop or Copy & Paste. This is useful when the text is inserted in another text, but it may prevent that iCab will accept it as valid registration key. So either check if there're no additional space characters appended or enter name and serial number manually.
The OK button will be activated as soon as name and serial number are entered correctly.