I never use a detergent either. If the kitchen floor is particularly grubby I slosh some normal floor cleaner (like Flash) straight onto the floor and steam over it with the pad.
Be careful what you put in as it can clog it up. I use ironing water (what you put in your irons) and it's cheap too, usually only a quid at local discount shop.
They actually give you a small bottle with the machine but I imagine it to be very expensive
Two good ideas - to apply directly to the stain and iron water
Thanks for those - and all your other answers