You can sign in two ways: a login code, or your email and password.
Before you start, make sure the iPad is online and your account has an active plan.
Login code (no password):
Open the app and choose the login option.
Tap Request Login Code and enter the account owner's email. You can add a phone number to get the code by text, which is easier to scan.
Scan the code with the iPad's front camera.
Email and password: use the same details as the dashboard. If you don't know a password (accounts created at checkout get a random one), use a login code or reset the password.
If it didn't work:
"Another device was signed out": your plan allows a limited number of iPads signed in at once. Signing in on a new iPad signs out an older one, which then shows "Session Ended". Sign that iPad back in if you still need it.
"Inactive Account": see "My account says inactive".
Reset email missing: see "Resetting your password".
Wrong email: the login-code message always says "If the email provided is an active account holder, it will receive a code to login shortly!", so a typo shows no error. Re-enter it carefully.