Hm, it appears the WP E-junkie plugin's developer has not updated that plugin since WordPress v3.0.5, so there may be some compatibility issues with WP v3.1 or higher. If the plugin does not work for you, we would recommend just pasting the button code directly into your WP posts. The button code provided in Seller Admin should automatically include fields/menus for products configured to use Variations/Variants. When you copy that button code to paste into your WP posts, you may need to save directly from raw HTML code view, rather than switching back to the visual-layout mode before saving.
Speaking of which, we can recommend the Raw HTML plugin to help get WP to retain your button codes as-is, exactly as you pasted them in:
Other optional editor plugins for WP may give you better HTML-editing capability than the standard WP post editor combined with the Raw HTML plugin.