The author abruptly yanked the support forum from the product URL after a couple folks couldn't figure out how to get it to work.
The Plugin does not fully work with 2.9.x, however the Widgets do work in the sidebar. The [shortcodes] do not work in pages/posts.
The Symbiosis mode does not work unless you place a copy of the configure table from the WordPress database into the OsCommerce database.
The documention on the site, what is there is excellent, however there seems to be stuff here, stuff there, and the support forum was full of unanswered posts.
Right near the end, the author said to expect a new version of WPosC around March 15th, totally based on WordPress framework.
But again, the whole support forum went 404, as well as the authors own WordPress page, it's also responding with a blank page.
Too bad, this is an excellent fork of osCommerce.