Looking at your css code, it appears you are using an outdated version of the plugin. Perhaps you downloaded the plugin while it was still in beta. You should delete the current version then download and install a fresh copy now.
OR…
If you have critical event data you don’t want to lose, try the following. Be careful, you could make a mess of your blog if you’re not comfortable using ftp. And I won’t be able to help you.
1. download the AEC plugin from www.ads-software.com directly to your computer (not via WordPress)
2. deactivate the AEC plugin
3. login to your host via ftp
4. delete the AEC plugin folder located in /plugins/ajax-event-calendar/
5. unzip the ajax-event-calendar.1.0.zip file downloaded from www.ads-software.com
6. copy the now unzipped ajax-event-calendar/ folder to the plugins/ folder on your host
7. activate the AEC plugin and you’re done!
This will make sure that you have completely replace the old version with the new version without losing your database events.
Good luck!