Go to your Account Settings and click Edit Appearance. Then select Edit Theme.
Underneath Custom Theme, click Edit HTML.
Copy and Paste Script
Paste the POWr script into the HTML file between the Head tags. Then click Update, and save.
New Text Post
Back inside the Tumblr editor, insert a new Text Element.
Copy and Paste Shortcode
Paste the POWr shortcode into the text post, then save If you expand the post, you will see your new POWr Mailing List. You can edit POWr Mailing List by clicking the POWr Settings Icon in the corner.
To edit your Mailing List on Tumblr:
Visit your live Tumblr web page and click the Settings Icon in the top right corner of your Mailing List Plugin.