It's difficult to customize the settings for the program's direct-to-web tools. However, if you use the two-step process of creating files and loading them to web sites, as described by John Cloudman in the FAQs to the right of this forum, you can customize them any way you want by clicking on the Advanced button under Share/Computer/MPEG.
BTW, I'm not sure why you're wanting to change the frame rate on your video. It's not going to give it a more filmic 24p look. (Even movies shot in 24 fps are usually converted to more standard 30 fps for display on DVD and online.) About the only thing it may do is reduce your file size a little and make your video's animation look a little stiffer -- though probably not noticeably so.