Ok, I couldn't get this thing to center no matter what I did :)
So, the fix is this -
At around line 14 of polaroid_gallery.css you will find:
clear: both !important;
position: relative !important;
margin: 40px !important;
padding: 0 !important;
Now, this depends on the width of your content div as well as how many columns you display BUT all you have to do is fiddle with the margin value. For me, I upped my columns to 4 and set it to 20px. When it was 3, it was closer to 90px. You get the idea.
Anyway, this plugin is resistant to actual centering so there's your workaround.
Hope this helps someone.....