Select your instance, in the "description" tab at the bottom of the page there is a "Security groups" section, click on the security group.
Then select the "Actions" dropdown and "Edit inbound rules" as shown below...
Now select "Add Rule" at the bottom. Set the type to "Custom TCP Protocol", the port range is "18010" (or the port you wish to open), and the source can be "anywhere" if you want anyone to access the /edx-studio or you can restrict it to "My IP" which means only you can access it. But, I recommend you choose this option only if you have a static IP as you will have to update it every time your IP changes.