June 2017 Update: Apple now requires that all third-party apps that sync with iCloud (like BusyCal) to use app-specific passwords. Please read these instructions for setting up an iCloud App-Specific password for BusyCal.

Apple offers an optional security enhancement for your iCloud account called two-step verification. Two-step verification requires you to verify your identity using one of your devices before you can connect to iCloud with a new device or app.

Apple's implementation of two-step verification is proprietary and only availble to Apple apps. If you're using two-step verification, you'll have to create an app-specific password for BusyCal or BusyContacts that lets you sync securely with iCloud.

App-Specific passwords are required for all third-party apps that sync with iCloud. Note: This only affects iCloud users who have enabled two-step verification. If you are not using two-step verification, you can ignore this.

These are the steps for creating an app-specific password for BusyCal and BusyContacts to sync with iCloud:

1. Sign in to your Apple ID account page.
2. In the Security section, click Edit.
3. Click Generate Password and follow the steps on your screen. Your password will be displayed on screen. Write down the password and/or copy it to your clipboard.
4. Enter the password in Busycal and/or BusyContacts.

In BusyCal or BusyContacts, select BusyCal or BusyContacts > Preferences > Acccounts, select your iCloud account in the left sidebar, then enter the new app-specific password in the Password field.