There is really no need for you to install this at all. The major search engines PREFER that URL re-writers are NOT used, as this can lead to errors in SE indexing, and many other problems, including duplicate content penalties.

Zencart is well-built, and renders the dynamic links just fine, and the major SE's navigate their way around ZC installations very quickly and easily.

Put your focus into logical and meaningful CONTENT, plenty of descriptive text, a valid robots.txt file and create a XML sitemaps submission interface with Google via Google Webmaster Tools.

Melanie Prough, a regular commentator on this forum and a respected SEO Guru, has excellent advice:-
http://pro-webs.net/blog/2009/09/10/...-seo-12-steps/