Strangely, the control to enable visitors to download Google+ photos is hidden in PICASAWEB settings. Wtf.
Go here: picasaweb.google.com/lh/settings#permissions. Under “Allow any visitor to:” check the box for “Download my photos”, then click save.
Your visitors after refreshing the album you sent them should be able to download now!
Ps. If you want to change your album cover, I found that the easiest way to do this is also through http://picasaweb.google.com/lh/ . Click into the photo you want to set. Then go to Actions > set as album cover.