The math diagnostic will start automatically for all new students when they first start using Sumdog.
If you have students who you would like to have retake their math diagnostic here is how you can reset their score. Once their scores have been reset they will be able to take the math diagnostic.
If you have set any work for your students, like a challenge or assessment, they will need to select their Math diagnostic from their Tasks list.
These instructions explain how students can take their math diagnostic.