If your account balance is $0 this is normal, do not be alarmed.
In the USA, you buy a plan which includes calls and/or SMS and/or data. You didn't buy "value" or "credit". To see what's included in your plan please click here.
So, for example, if you purchased the USA Extreme Plan for $89, then there remains a $0 residual balance. Think of it like buying a loaf of bread for $3. You give the baker $3, and so how much do you have left? (hint: $0!, but you now have a loaf of bread!)
If you're trying to call Australia and you get a message saying you don't have credit, please click here to read more.