Using the Featured Posts Grid in Portfolio Press Theme

Hi there,
I am modifying a website using the Portfolio Press theme, and we have a page with posts already on it. However, I want to convert the posts byhaving a thumbnail link to them. I have found the “Featured Posts Grid” & have installed it on the site, but does not work properly. Can anyone provide me with info to getting the plugin to work in this theme and/or knowing where in the theme to insert the relevant CSS code into it to get it working. Also, any other suggestions for accompolishing this task would be much appreciated!
Thank You.