Probably because it’s a non-standard function, but I’m not sure… But you can use to_string in C++11 or you can still write your own itoa function, which is not that difficult.
As I said, if you’re out of ideas, you can still write your own itoa function. It should look something like this: http://ideone.com/CCVrh6
It’s probably not the smartest implementation, but it should give you an idea, if you hadn’t yet figured it out.