First, open your Tumblr blog (not your Tumblr feed) in your browser when you're logged in to Tumblr. Next, click on the small 'Edit Theme' button. It's on the right when you hover over your Tumblr site. This button will open your theme editor.
Click the 'Edit HTML' button on your theme to open the HTML editor. Once this is open you should see the code for your site.
Login here to get your actual Plot script code to paste into your blog. You can also copy it from your project settings page in your feed.
Paste your Plot script into your blog's HTML between the
</head>tags. Anywhere in the page head will do.
After you pasted in your code, press the 'Update Preview' button in the top right of the HTML editor for your blog. After updating, press the blue save button and click the back arrow to leave the HTML code editor.