The have a new version of SourceTree Mac v2.2.2(51) / Win: v1.8.2 and when you have this version you can customize you toolbar by clicking on the toolbar with your right mouse button. Then you can enable the git flow button.
Jens Schumacher has just wrote that the git-flow button will be back in the next bugfix release. As for now, as others suggested, either roll back to a previous version, or use a hotkey (Alt+Shift+F for Windows, ⌘+⌥+F for OS X).
But, also you can configure the toolbar to show the gitflow icon again. You just have to do a right click on the toolbar, select "customize toolbar..." and drag the icon back to the place it use to be. As simple as that.