Actually, please read this (from the FAQ):
Each time WordPress Popular Posts is updated the wpp.css stylesheet gets reset and I lose all changes I made to it. How can I keep my custom CSS?
Copy your modified wpp.css file to your theme's folder, otherwise my plugin will use the one bundled with it by default.
Also read this (from the Description tab):
- Minimum requirements changed: version 2.3.3 (and on) requires PHP 5.2+ and WordPress 3.3 (or greater).
- The custom HTML layout functionality has changed on v2.3.3! WPP will attempt to use the markup you set on previous versions, but in case it breaks please check the FAQ section to learn how to set it up again.
- From version 2.0 and on, WordPress Popular Posts requires WordPress 2.8 at least in order to function correctly. If you're not running WordPress 2.8 (or newer) please use WordPress Popular Posts v.1.5.1 instead.
- If you are upgrading from any version prior to WordPress Popular Posts 1.4.6, please update to 1.4.6 first!
Finally, please read this - and pay special attention to the last line:
I need help with your plugin! What should I do?
First thing to do is read both FAQ and Installation sections as they should address most of the questions you might have about this plugin (and even more info can be found via wp-admin > Settings > WordPress Popular Posts > FAQ). If you're having problems with WPP, my first suggestion would be try disabling all other plugins and then re-enable each one to make sure there are no conflicts. Also, try switching to a different theme and see if the issue persists. Checking the Support Forum is also a good idea as chances are that someone else has already posted something about it (and if not, you are always welcome to create a new thread). Remember: read first. It'll save you (and me) time.