I answered this in another thread already. CC does not recognize environment/path variables in a string. That's what the path object is for. Add it to your project and retrieve the app data path by using the expression "Get App Data".
You can get all system paths by using the path object.
In your case the path string would be build this way:
Path.GetAppData & ".minecraft\config\file.txt"
And don't forget, it's a backslash, not a slash.