WordPress.org

Unpack the .zip file onto your hard drive.

Upload the mangapress folder to the /wp-content/plugins/ directory.

Activate the plugin through the 'Plugins' menu in WordPress

Create two new pages; these pages will be your latest comic and comic archives pages. Label them something like 'Latest Comic' and 'Past Comics' or whatever, as long as it makes sense to you.

Click on the Manga+Press Options tab under Settings and go to Basic Manga+Press Options, and set Latest Comic Page, and Comic Archive Page to your two newly created pages.