The first thing is to understand that, the Dispatcher is not designed to run long blocking operation (such as retrieving data from a WebServer...). You can use the Dispatcher when you want to run an operation that will be executed on the UI thread (such as updating the value of a progress bar).
What you can do is to retrieve your data in a background worker and use the ReportProgress method to propagate changes in the UI thread.
If you really need to use the Dispatcher directly, it's pretty simple:
Application.Current.Dispatcher.BeginInvoke(
DispatcherPriority.Background,
new Action(() => this.progressBar.Value = 50));