Hi Sujit,
After you created your sandbox district, were you able to populate it with the data from the CSV files? Within the collection of CSVs you'll find a teachers.csv and a students.csv -- each file contains usernames and passwords that you can use while signing in with Instant Login with your sandbox district.
If you sign in to your district dashboard, click "Browse" under Data in the right hand rail and click on the name of your demo district in the resulting screen, you'll see a pop-up that contains that district's Clever ID.
Thanks,
Taylor