AutoCAD 2015 and its verticals do not include the horizontal scroll bar for viewports. Personally, this is a feature I have never used since it was introduced, and in fact is one of the first things I disable in a new install. But one of the quickest ways to find out if something is being used it to take it away, and there are obviously a lot of people who prefer to have the horizontal scroll bar.
The current Autodesk Knowledge Network article on this topic originally did not offer much help other than to reinforce the fact that the horizontal scroll bar is not available. A few weeks later, they did update the document to say that you can use the middle mouse button and the Shift key. It should be noted that the -Pan command with Ortho can also constrain the movement to horizontal or vertical.
Update: Installation of AutoCAD 2015 SP2 should restore the horizontal scroll bar.