The Terrain I want to model is outside the USGS data set, what can I do.

There is a ‘User’ folder of the Cache folder used by the plugin. You can download DEM data files and place them in that folder, the plugin will check this folder first for the data it needs. You can use this to add higher res DEM data if you have it as well, for example the USGS data only covers areas outside the US with 90m data but you can easily find 30m data on the web and download the files you need to the user folder. One place to get data is http://www.viewfinderpanoramas.org/Coverage%20map%20viewfinderpanoramas_org3.htm