ARM LPC1751 pins configured as  I/O
If you don't find the complete information in the datasheet you'll have to look at the LPC17xx family user manual. We're interested in Chapter 9: "LPC17xx General Purpose Input/Output (GPIO)". There you can read that FIODIR is the 


  Fast GPIO Port Direction control register. This register individually controls the direction of each port pin.  


The direction for each I/O port/pin is controlled by FIO0DIR through FIO4DIR. Page 132 gives you the corresponding addresses.

If you don't know how to start looking in these rather long documents, use the "find" function in Acrobat Reader (CTRL-F) and look for "Direction".