By default when students first start on Sumdog they will be in Training mode. This is where they can work on their diagnostic test and work through the skill plan.
If you have set an assessment, challenge or competition for your students or if they are playing in the Sumdog contest they will be able to select those from their task list.
To choose which activity the student would like to work on they can click on the tasks list on the left side of the screen.
Whatever task title is on the student's screen is the one they are doing. Click 'start' to either choose a game or to answer questions in an assessment.