Serin is for asynchronous serial communication and can't be used with that shiftregister. You need to use Shiftin which is for synchronous serial communication.

With the Shiftin command you specify two pins, one for the data and one for the clock. First you need to load/latch the data into the shiftregister by pulsing the PL pin, then the Shiftin command is used to shift data out of the '165 and into the PIC.