You are not rooted. Therefore, you have no use for Superuser.apk, even if you were able to install it from the market. It's like hiring a babysitter without having a kid, then wondering why she leaves shortly after you call her over to your house every time. Superuser.apk is a permissions management GUI, similar to UAC on Windows or gksudo on Linux/Mac; it limits access to administrative functions when you have administrator rights. This is different from the su binary that, when installed, allows you to perform the administrative or root user tasks.
Having the Reborn ROM zip on your SD card is nice, but does nothing except take up space unless flashed via a custom recovery on your rooted phone.
Solution: You need to root your phone using the method appropriate to which ever baseband you are on. ZVD is arguably fewer steps, but you have to use works with the setup you have. Then you can install your custom recovery, do a backup, flash a ROM, etc. See the Welcome Gude already.