If you would like to add a tag to a Lead and their profile, follow these steps.
Navigate to the Lead list (located on the left-side navigation bar, entitled "Lead List").
Find the Lead you want to tag and click on them (this will take you to their profile)
You can click “write a note” and there you will see an option to tag (you do not have to write or note. Or you can click on the “Info” tab and click the + (plus sign) in the upper right corner of the “Tags” box.
There you will see the tags available to you and you can click on those that you want to add to that Lead. You will know which you have selected because they will turn a color instead of being transparent.