The script from Google AdSense you should put in the "Ad Manager" section.
Just create some new Ad using the according the "Add new" button:
On the Ad Add/Edit screen put the script from Google AdSense in Text mode and choose the banner/banners.
The rest of the options like Ad start/end days, Exclude posts by ID, etc. you should configure as you need.
Regarding the "Ad Banners" section. Here you can create the Banners and choose the banner location.
In other words, you'd need to create the Banner with its name and location, then put your custom Ad text in the "Ad Manager" section and link it with the Banner/Banners you've created.