It is available on PIC18F processors. To enable it, click on "View" in your PBP header, then select "Compile & Program Options...". In the pop-up window select the checkbox "Use Compiler Long Words (18 Series MCU Only)"

Once activated, it is on all the time (until you turn it back off). If you try to work with a PIC16F processor, you will get compiler errors because of the Long. In other words, turn it on when you need it, and turn it back off when you're done.