1. If you have miui downloaded open the zip file with WinRAR (or 7zip)
2. Then go to system>app, move the camera.apk to your desktop
3. Move it to your SD card
4. Using ES File Explorer enable Root and Mount System in Es File Explorer's settings
5. Copy the camera.apk you put on your desktop to your SD card
6. Push the star button and push the phone icon it should to you to the phones root
7. Go to system>app and paste the camera.apk here
8. Press and hold the new file to bring up a menu and scroll down and select properties
9. You should be able to change permissions here.... change them to rw-r-r
10. Finally reboot and it should work
Other option would be to move it into the Rom you want it in before you flash it.....
PS. You might have to delete the old camera.apk before you copy over the new one most-likely not though.......