To install and play packs on ATLauncher, you need to add in your Minecraft account to the launcher. This must be a Java edition of Minecraft, and you cannot use a Windows 10, bedrock or trial edition of Minecraft to use ATLauncher.
If you do not have a paid for Java Minecraft account, you can buy one here. ATLauncher doesn't work for cracked/free accounts.
If you're worried about entering your login information into ATLauncher, we're completely open source and our code is all available on GitHub for all to see.
If you've purchased Minecraft before the 1st of December, 2020, you'll have a Mojang account (unless you've migrated). In this case, you can login using your username/email and password as below:
For accounts purchased after 1st of December, 2020, they're associated with your Microsoft account. You can login to your account by clicking the Login with Microsoft button. Clicking this will open your web browser to complete the login process with Microsoft: