After you install the app, and have your photo ready, choose File, Export>ApertureToSmugMug

 
 
Then put in your SmugMug login credentials (username is same as nickname, or you can put in your email address).
You can add caption & keyword info here as well, you can choose the album, too.
 

 
 
For preset, be sure to choose JPEG.  Originals are best, but you can upload smaller... but BEWARE that if you are printing or selling prints,  your photo must meet the resolution requirements here.
 

 
 
Click Export and watch 'em go up to SmugMug