Reinstall TouchWiz Calendar I've been trying to reinstall the TouchWiz calendar on my Sidekick 4G with no luck.

I got the calendar apk here, and put it on my sd card then moved it to system/app using RootExplorer, and once I clicked install I got the normal install prompts then a message that said "Application not installed".

How can I fix this? Do I need to reinstall a stock ROM?
You don't install system apps, they're automatically detected.  After you move it to /system/app/ just remount system as read-only and reboot.  If your ROM's not deodexed that may not work but that's a whole other can of worms.