In my albeit limited experience, Betfair's mobile java program can often be a little troublesome to set up and only works on certain mobile phones.
I've had a much better luck with Betfair lite which is a cut down version of the main Betfair site, but specially designed for mobile phone web browsers to access.
As nearly all modern mobile phones can access the Internet via GPRS etc, it's a simple case of pointing your mobile's web browser in the direction of
http://lite.betfair.com/