Just a quick re-send of info from last year, I could not find this on any USGS web sites when I was helping someone, so thought others may have a hard time, too: 

The how-to : http://www.fdlp.gov/all-newsletters/community-insights/2045-tricks-and-tips-for-finding-and-using-usgs-topographic-maps

USGS-ESRI press release, July 2014

http://www.esri.com/esri-news/releases/14-3qtr/new-york-to-la-history-of-americas-maps-in-one-app 

Enjoy the day,

Emily