Dennis, you may be new at some thing, but building web sites is not one of them.
New at dropping links in this forum, maybe, but not building web sites.
I have always been able to paste the Google code directly into the themes where I want the ads to appear and have them work. Your method of a separate file and the include function would work as well or try this pugin AdSense-Deluxe: https://www.acmetech.com/blog/adsense-deluxe/.
