The next/previous navigation links in projects are something that I am planning to add to the theme in version 3.3 (in fact, their “prototype” code, although disabled, is present in 3.2 as well), yet I haven’t yet finalized it. I can send you the template file with the functioning links, which will add next/previous links to each portfolio project; please note, however, that the previous_post_link and the next_post_link WordPress functions only allow for chronological order, i.e. the next link will point to the item which was created (chronologically) after the current one, which means that the order of the nav links will coincide with the order of the tiles on the masonry page only if the “Order of portfolio items” option in the “Portfolio Page” tab of the Theme Options panel has been set to default or “Date created”.
In any case, here’s the file, please download and unzip it, and upload the resulting file via FTP into the theme folder: /wp-content/themes/Ikebana/, replacing the existing file.
Let me know if you face problems with the file, or in case any other questions pop up!