Log into www.addthis.com. Click the link at the top of the page named "Grab the code". Select the style on the left, then copy the code in yellow. Image below shows the same screen.
After you have the code copied to you clipboard, log into your Pressero admin account. Choose the site in the drop down, then go to settings page, the the SEO/HTML tab, finally open up the "share script area". Paste the code within the share script area.
Once you have pasted the code like I have in the image above, scroll down the page and click save. Your site will not update instantly, you will have to clear the cache in order to see the changes right away, or you can wait about 20minute for the cache to cycle itself.
To clear the cache for any Pressero site just type in the site URL into any browser address bar. Followed by /clearcache. You will be presented with a page that has a clear cache button. Click that button. Here is an example "http://www.pressero.com/clearcache".
Now the server cache between Pressero admin and your site is clear and once you refresh your site you should see the add this buttons like in the image below.