I came across a handy way to make sure the syntax you use for this path is exactly right: open up package configurations and go through that wizard selecting the value you want the path for, on the "Completing the Wizard" summary screen you'll see it under "properties:". Also, there's more than one "correct" path. In my case I guessed the first and generated the second of these:

\Package\Data Flow Task.Variables[QueryText].Value

\Package\Data Flow Task.Variables[User::QueryText].Properties[Value]