This should all be obvious after a single glance in the plugin source code you linked!
While it shouldn't crash (after a quick look into the class code), you probably do not supply the appropriate sender when sending buttonPressed: from your code. The method is defined as
- (void)buttonPressed:(SFButtonType)inButtonType
Which clearly shows the button that was pressed is given as sender. So in your code when calling buttonPressed:
do for example like:
["referenceRToClass" buttonPressed:SFButtonTypeOK];