Okay, this might be a simple question that I'm somehow missing, but how do you simply add php calls to a zenphoto page? How do I edit a page? I'm trying to add a calendar to my archive page but I don't see 'archive' anywhere in zp-core. All I see in my browser is www.mysite.com/page/archive. Where is this generated from and how do I edit it?

Also, how do I add the 'popular tags' box from the archive page to my main gallery page? Thanks!

The zp-core folder is the wrong place to look.
What you want is located in your theme folder. That's where you'll find [i]archive.php[/i].

The function to use for the 'popular tags' is printAllTagsAs().
You can simply place this function where you want the tags to appear.
Info on how to use this function and how to set parameters for it can be found here: http://www.zenphoto.org/documentation/functions/_template-functions.php.html#functionprintAllTagsAs

Best you read the theming tutorial to get more familiar with customizing Zenphoto.
http://www.zenphoto.org/news/theming-tutorial