No, you can't make any changes. I called and emailed to see if I could but I was told that no changes are allowed after submission.
You can update the schools directly if you think the activity is valuable to your application.
Eh it's ok. It wouldn't make a difference anyways. It's just that I mention the activity in my secondaries for Southwestern, A&M, and Texas Tech at Lubbock and it is one of my most meaningful activities under AMCAS. I can't believe that I didn't notice that I forgot it. I'm pretty sure that I typed it in, but I forgot to click Save and Continue and didn't notice it when I double checked everything.