Pinmode register in lpc2468
Pull-up and pull-down apply to I/O used as input, not output. Inputs shouldn't remain floating, so you either  


drive them with a push-pull output to a logical low or high level
use external pull-up or pull-down
use internal pull-up or pull-down  


Pull-up/pull-down will set the input to a valid level if it would otherwise be floating. The second and third option are normally used with either open drain outputs (usually with pull-ups) or with switches/pushbuttons.

Remember that the datasheet will in general only give you device-specific information. Data which is common to all members of a microcontroller family can be found in that family's user guide.

Further reading
LPC24xx User Guide
LPC2468 Datasheet