Never did it myself either, just looked at can't seem to see how it's done... Try opening Betfair in safari rather than through the app itself or else ring them 1800944000 I'm sure they'll be more than happy help you deposit
There are a couple of apps Daz. I use one called ibet mate. You can't withdraw or deposit on it though. On the official app you can't deposit or withdraw either although its a bit fancier than the one above. If you go into the safari browser and go to www.betfair.com, it comes up in all iPhone friendly interface and you can deposit and withdraw if you login through exchange section