Ok folks I've figured this out, so here's my PSA:
Aspen GO is not an app, it's a mobile-friendly version of the Aspen website. You can access it via a mobile device here:
https://aspen.dcps.dc.gov/aspen-login/go
Then you use your usual Aspen credentials to log-in. There are links if you have forgotten your credentials.
Once you have access, here are the instructions for submitting an excuse note within the system:
https://dcps.dc.gov/sites/default/files/dc/sites/dcps/page_content/attachments/Submitting%20an%20Excuse%20Note%20in%20Aspen%20GO%20%28Parent%20Portal%29.pdf
(This link was buried on the DCPS site amidst a long list of other attendance-related PDFs, you should not feel bad if you didn't see it!)
But anyway, none of this matters because if you follow the directions in the PDF, they tell you to go to "To Re-enroll student and other forms. Totally non-intuitive since most people finished re-enrollment months ago and I would not have expected the excuse notes to be categorized under "and other forms" because I would assume this just refers to enrollment-related forms, not attendance-related forms. I would expect an attendance-related form to be available in the menu item labeled "Attendance." But of course, it's not.
But in any case, they have not yet updated the system, at least for my kids, because even when I followed the directions I still didn't get an option for excuse notes. So I guess we'll see if/when that gets fixed.
Also the mobile site is slow to load even though the whole point of a mobile version is to have a slimmed down version that is easy to load and navigate on a phone.
Do better, DCPS. For once.