After the EC2 instance is started, you can access the RStudio Server with its public IP address. For example, if the IP address is 10.11.12.13, typing http://10.11.12.13 in the browser will bring you to the following startup page:
The default username is ruser and the password is the last 12 digits of the vmId. The vmId can be found in the JSON View with the form of 1f61b751-037a-489e-8638-3fc32fe6894d. Only the last 12 digits 3fc32fe6894d are used as the password.