how to define waveport for slot line to microstripline transition

hi
i want to assign wave ports to two sides of slot line to microstripline transiotion. but i got an error msg every time that port is assigned to an internal face which is only allowable to lumped port.what does this error mean and how can i remove this error.i am attaching the paper from which i am trying to made that balun plz guide me.......