On your Pinterest Settings page, click to verify your site and then click "Verify with a meta tag."
When you have your meta tag code, just go to Customize Your Design, then click your Code tab, and paste it inside your site's "head," or just before you see this line of code:
Then, go back to Pinterest and click the link to complete the process. You should see that your site has been verified. If not, double-check that you've placed your code in the right place.