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 labeled Sign In.
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.
After logging in, if you are a user in multiple MC Trade database instances with the same email address, select the same of the association at the top of the page and a new window to switch databases will open to allow you to enter the name of other databases to switch to.