As soon as you add your site's link to GWT, you'll be asked to verify your site.
Click "Verify Your Site", choose "Add a meta tag" as the method of verification from dropdown-box.
Copy the code.
Now simply go to your bloggers control pannel, go to 'LAYOUT', then to 'EDIT HTML'. Find the word <head>, paste the code just below it. Make sure that you paste the code just after opening tag, not after closing tag. After this, come to GWT and click the verify button.