You can either go to Roku's Account Dashboard, login, and add your channels there. (Channel Store)
https://channelstore.roku.com/browse#!
or...
From your Roku Express Home page, scroll down to Search or Streaming Channels, and select Search Channels. Then search for Amazon Prime or Disney+ from there.
Some of these channels are region specific so may not be available. Disney+ is expanding to Latin America but not available everywhere yet, so that may be your issue.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102