how can i add the download folder to iphoto=sidebar on a 2015 macbook air [with high sierra]

"Add the download[s] folder"???

That seems like an unusual request (to say the least). Here's why:

• If you were going to add a folder, the "Pictures" folder seems much more logical than the "Downloads" folder. The "Downloads" folder may contain many items which are not pictures or photos.
• The Photos sidebar displays items unrelated to folders, and does not include anything related to folders, or the data organization on your drive. Instead, the items are related to data organization within the Photos app. In particular, the items displayed (major categories) are:
  • Library
  • Shared
  • Albums
  • Projects

iPhoto was discontinued by Apple in April, 2015 with the release of OS X Yosemite 10.10.3.

It's time to move on to Photos.