WordPress.org

The visual editor replaces div-tags with p-tags

If you're using the visual editor the div-tags will be replaced with p-tags. To keep the markup intact you'll have to disable the visual editor if you'd like to edit a post/page that contains a SmoothGallery.

To do this, go to Users -> Your profile and uncheck "Use the visual editor when writing". Once you've saved and published the post/page you can activate the visual editor again - but make sure to disable it again if you want to change a post that contains a SmoothGallery.

The SmoothGallery doesn't show up in Internet Explorer

SmoothGallery doesn't seem to work with Internet Explorer if you use Prototype on your site.

Read about a solution here. Basically you'll have to make sure that you don't use Prototype and SmoothGallery on the same site.

What about integrating it into my theme?

Read about this here.