After trial and error, I finally found the right combination for Mantra (at least 188.8.131.52):
Content Selector: #content
Posts Selector: #content article.post
Navigation Selector: #nav-below
Next Page Selector: #nav-below .nav-previous a:first
Please add it to the preset base. It works perfectly, as you can see at http://kostyrka.ru/blog/. Thank you for your awesome plugin!
Andreï Victorovitch Kostyrka, Russia, Moscow.