This happens because Google has detected the bot crawling it and automatically blocks searches coming from that IP for a period of time. When SearchDiggity was created that time period was exactly 14 minutes, so the tool waits 15 minutes to be unblocked. Check out this blog entry for more details and see how to bypass this issue: http://www.bishopfox.com/blog/2014/08/searchdiggity-avoid-bot-detection-issues-leveraging-google-bing-shodan-apis/
As far as using SearchDiggity, the application's main web page is filled with useful information, so I recommend checking it out: http://www.bishopfox.com/resources/tools/google-hacking-diggity/attack-tools/
Lastly, according to the same source, the tool's last update came in 2013, so I think it's safe to assume that the application is no longer in the development phase.