BackStreet Browser starts download automatically when you confirm a new project creation by pressing the button "OK" in the Project properties dialog.
If the download was interrupted, you can restart it by pressing the button "Start" in the application toolbar (you can also use the main menu item Project\Start download, shortcut F9). BackStreet Browser will offer you a choice: to update the files that have already been downloaded, to restart the download process from the very beginning or to resume it and restart from the point where it was stopped before.