In order to open an exercise system you need to enroll in the course. Once enrolled you will start progressing through the course, most not all lessons have an exercise associate with them. For each exercise there is an Operator ID/Password combination. At the bottom of the exercise screen you will see a link to Open exercise system.
For example, in the System Architect Essentials course on the Designing a Case Lifecycle the Operator ID/Password combination is author@tgb/rules.
Please pay attention to each exercise as the Operator ID/Password may change.