No horizontal scrollbar in Xcode's Interface Builder
Press Shift + Scroll Mouse this will scroll preview in horizontal
It's a not bug, it's a feature. It wouldn't be a problem if you use the Apple mouse. There is a workaround for a typical 2/3 button mouse, although it would affect all horizontal bars across the system. Just follow System Preferences->General->Show scroll bars, and select Always.
I have been struggling with this as well. The only solution I found is the following:
So there is a mini map for the storyboard that hovers over the right upper-side.
If you can't see it, just click on the "Adjust Editor Options" button and make sure Minimap is selected.
This is the best way to move around just like you are using the scroll bars.