Just type a number into the scratchpad and line-select it to the line on which the T/O shift has to be entered?!
Depends on your FMC software whether for e.g. 400 metres you only need to enter the "4"', since the "00" seem to be in place already, or whether you have to type 400. Just try it during your next flight ground operations - you can always delete the figure again, or enter a displacement of 0.
Anyway, the distance to be entered is "down the runway", so (+) 400 metres will mean that your actual runway available is 400 m shorter than full length. The FMC only uses this info for a better update of position when you initiate the take-off (it does not change FMC calculated take-off speeds). The distance down the runway can be determined from your airport chart, if necessary, together with the "available distance" signs that should be provided at RW intersection points.