How do I approve my Mac from my iPhone?

Have a look at this article: Set up iCloud Keychain

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To use iCloud Keychain, you need iOS 7.0.3 or later on your iPhone, iPad, or iPod touch, and OS X Mavericks 10.9 or later on your Mac. Learn about availability by country or region.

When you update your iPhone, iPad, or iPod touch to the latest iOS, the setup assistant asks you to set up iCloud Keychain. Instructions vary based on whether you're using two-factor authentication or another type of security, like your email, password, or security questions.

If you're using two-factor authentication

With two-factor authentication, your device is trusted when you sign in with your Apple ID, so in most cases you don't need to re-approve it.
On your iPhone, iPad, or iPod touch:

1. Tap Settings > [your name] > iCloud. For 10.2 or earlier, tap Settings > iCloud.
2. Tap Keychain.
3. Slide to turn on iCloud Keychain.

On your Mac:

1. Choose Apple () menu > System Preferences > iCloud.
2. Select Keychain.

If you're using two-step verification, or email, password, or security questions

On your iPhone, iPad, or iPod touch:

1. Choose Settings > [your name] > iCloud. For 10.2 or earlier, tap Settings > iCloud.
2. Tap Keychain and slide to turn it on.
3. Enter your Apple ID, and follow the instructions.

On your Mac:

1. Choose Apple () menu > System Preferences > iCloud.
2. Enter your Apple ID and password, and follow the instructions.

You can create an iCloud Security Code—six digits, complex alphanumerics, or randomly generated—to authorize additional devices and verify your identity. If you have the SMS-enabled device that you used to set up keychain, you can use that, or you can get help if you forgot your code or can't get the SMS. You can also change your trusted phone number in Settings or System Preferences.