It looks like there are some conflicts with the existing CSS you already have on the page. You can debug the exact problem by using Firebug in Firefox or the Developer tools in Chrome/Safari.
For the gallery navigation specifically it looks like you have a .nav a CSS rule that is conflicting with the .nav class defined for the Powergallery design. Another way to troubleshoot would be to construct you page with a gallery first, then add in your custom CSS a little at a time so you can pinpoint exactly what it causing the issue.
-justin