Uploading more than one thumbnail picture at a time?

You need the flash uploader, which is the default upload method. If you don’t have flash, it falls back to single file upload. You probably don’t have the flash plugin installed in your browser.

Where can I download the flash uploader plugin?

It’s not a plugin, it’s built in. As of WP 2.5 and up, the flash player is the very first thing to launch when you use the media uploader. If you do not have flash installed on your browser, it won’t show up. Go here: https://www.adobe.com/shockwave/download/download.cgi?P1_Prod_Version=ShockwaveFlash