You'll notice in the Search Window it says Search for layers. Maps for Office in Excel can bring in feature services and layer content, but not webmaps. Once you create your own layers and add any additional ones from AGOL, you can click the "Share Map" button in Excel, and it will package all of the layers into a webmap. Maps for Office in Powerpoint reads webmaps and can create static and dynamic slides from them. Does this help to clarify?