Need help centering submit buttons and submit images...

I've been searching through the forum and going through different threads on this topic and the page at http://www.gravityhelp.com/documentation/page/CSS_Targeting_Samples, but cannot seem to get this to work..

Item 1: I have form setup at http://www.batonrougesummercamp.com/register and I'm using an image as the submit button. Trying to make it centered on page, and have added the following code to my theme's (i.e. Optimizepress) css file. See http://screencast.com/t/WzxQzJuxKH but still not working..

Item 2: I have form setup at http://www.batonrougesummercamp.com/pay which uses the standard submit button, but same deal.. Cannot seem to make it centered on bottom of the page..

Any ideas what I'm doing wrong?

Thanks for your help Rob... I added the code to the bottom of my theme's CSS file. See http://screencast.com/t/9LJ5KeKm

Still having same issue on buttons, even after I cleared my cache/deleted cookies, etc..

Any other ideas on what I could try?