Wow it used to be a mission - when it first came out, I would have to call 10219 and ask them there - after going through a very necessary lengthly security check... all for an amount of free minutes available
Needless to say I haven't bothered since - I find it tough to exceed the bundled minutes.
Not sure if they've improved on things now though maybe?
It is available under online billing - I think under account details/balance and then you either select outstanding balance or free minutes. I do not seem able to get access at the moment so can not check - will try again later if I remember.