Are you loading this through the Flash IDE or testing it
live? This tutorial seems to be doing exactly what you want to do:
How to
use LoadVars Tutorial
It could also be that you have to do something the fact that
you're not "swonking"- you have to wait for Flash to finish loading
the data in, otherwise you essentially end up calling a method on
said data before it's loaded as Flash doesn't perform a
method/function and then wait it will carry on- which I expect is
happening to you. Look at the tutorial above, it shows you how to
combat this problem by creating a delegate function which will wait
for your data and then try to use.