It's most likely a CSS issue and possibly a bit of markup that needs to be adapted to fit your theme. Since the output of the Posts in Page plugin is unstyled, it relies on your theme's built-in styles for formatting. In order for this to work, though, the markup and styles for your theme need to be applied to your Posts in Page template.
You'll most likely need to customize the posts_loop_template.php file to match your theme's markup and CSS.
We'll be posting an updated version of this plugin soon and plan to add a few screencasts with instructions on cutomizing, but in the meantime, please feel free to post links to your sites after you check out answers to similar questions in these threads:
- http://wordpress.org/support/topic/how-to-add-featured-image?replies=16 - this is about Featured Images, but explains how to customize your posts_loop_template.php
Styling Posts in Page Output to Match Your Theme
I recommend using Firebug to take a look at the markup and styles that are being applied to your existing pages and posts and tailor your CSS for the Posts in Page markup to match.
Also, if you wish, you can could copy the
posts_loop_template.php file from the plugin directory to your theme's directory and tailor the markup to match your theme's markup.
I hope this helps!