I am receiving the error message: the plugin doesn't have a valid header and can't activate the plugin. What am I doing wrong/how do I fix this? Downloaded Zen 1.3.

Maybe you should provide some info. For example what plugin and the correct error message from your server's error log.

I received this error message when I uploaded the Zenphoto 1.3 wordpress plugin using the upload plugin function in Wordpress 3.1. The files were loaded on the server, but the plugin doesn't show on the plugin list to activate.

What server messages am I looking for?

Sorry, this seems to be a Wordpress question. Perhaps you should ask on that forum.

It's an error when Wordpress tries to load Zenphoto 3.1 from the zip file. How do you recommend loading and activating the plugin in Wordpress? I tried loading the zenphoto directory to the plugin directory, but wordpress didn't see the plugin to activate it.

By the way, here's a site that describes the error and why it occurs (has to do with thee way the zip file is created). http://autoblogged.com/kb/installation/invalid-header/

Perhaps there is some confusion here. Zenphoto is NOT a Wordpress plugin. Nor can you run Zenphoto from the zip file.

Here is a quote from the link you provide:

This error is caused when you use the WordPress upload feature to install a plugin from the Add New plugin page. Unfortunately, this feature cannot handle a zip file that contains folders so it places the files in a subdirectory where WordPress cannot see the plugin header files to activate the plugin.

So clearly if there are folders within the zip file it is not a valid Wordpress plugin.

BTW, your site does not use Zenphoto. I am beginning to think this post is a subtile SPAM attempt. If you do not convince me otherwise the post and your user ID will be removed.

Zenphoto is listed as a gallery script on Wordpress's site...http://codex.wordpress.org/Photoblogs_and_Galleries. Sorry about the confusion...

It is a "gallery script". But it is not listed as a Wordpress plugin or in that section, right? A quick look at Zenphoto.org would have told you that as well...

However, it is possible to use Zenphoto togehter with Wordpress and there are some plugins for Wordpress that help with that (we use it currently together as well, although separately). Those are quite easy to find on our extensions section:
http://www.zenphoto.org/category/extensions/wordpress-tools/