Once your system administrator has set up a new user instance for you in your organization's database, you can log in at https://mctradeams.com. (You'll notice this resolve to app.atlasams.com - if that link is bookmarked, it will work fine too.)
If you're already logged in, that link will just take you right on in to MC Trade and you can get to work! However, if you haven't logged in recently, you'll see a blue button, Sign In to MC Trade:
Click this button, then enter your username (or email address) and your password, then click Login.
You'll be authenticated to your organization's database and land on your Dashboard.
If you manage more than one MC Trade database, you can dictate which one you want to log in to even if you use the same email/login for each one on the sign in page. Before clicking the button, just click "If you have multiple MC Trade databases, click here to enter an instance name." A text box will appear, and you need to enter the instance ID provided to you by our Help or Onboarding team for the database you want to log in to. Then click Sign In to MC Trade, and you'll be taken to the login screen specifically for that database.