Sheldon,

What's happening there is ... it will look at the state of the specified pin, and use either 0 or 1 for the pulsin pin, which is usually PORTB.0 or PORTB.1 depending on the chip being used.

So let's say T = 3, then the LOOKUP returns 16.
An offset of 16 from PORTA.0 is PORTC.0, so it gets the state of that pin (0 or 1).
If PORTC.0 = 0, it does the PULSIN on PORTB.0.
If PORTC.0 = 1, it does the PULSIN on PORTB.1.

There's probably a hundred ways to accomplish it, but here's an untested/not compiled thought.
Code:
FOR T = 0 TO 17
    lookup T [ 9,11,13,16,18,22,1,4,6,8,26,27,28,25,23,21,32,34], Portoffset  ' Sensor input 1 - 18
    Leader = 0
    IF PORTA.0[PortOffset] = 0 THEN
        WHILE PORTA.0[PortOffset] = 0 : WEND  ; Wait for start of pulse
        WHILE PORTA.0[PortOffset] = 1         ; Measure Pulse Width
           Leader = Leader + 1
           PAUSEUS 5                          ; Adjust loop timing to your needs
        WEND
    ELSE
        ; error, pin was not low to start
    ENDIF

    Sensor(T) = Leader                        ; Store Result
NEXT T