I don't have camera or manual with me at the minute but its fairly simple (otherwise I'd have had no chance at 30+m on the Eddystone earlier this year
)
When you're in photo mode, press the OK/Menu button in the middle of the 4 arrows and you'll get a menu with 4 options, one of which should be WB (I think its the bottom option). Select this and you'll be taken to a screen where you can choose from various pre-set white balance settings, along with the final option to set a custom option ( I think it has an icon that looks something like >[]< . Select the final option, then find something white (I take a slate in my BC pocket just for this), point the camera at it and press OK.