Hi russet (mind if I call you "russet" for short?
) welcome to android forums!
I think we've got some root confusion here, so I think I know what your asking, but if I'm off just whack me across the snout with a rolled up newspaper.
I think what you are looking to do in rooting your phone is to flash the su.zip (or whatever the filename is for your particular version) to root your S2. I'll bet the instructions tell you to put it in the root of your SD card, hence the confusion.
"Root" is both an account and a location. As an account it is the system administrative account that has access to everything. By gaining root on any Android ('nix) operating system you have the ability to modify or delete system files and folders and have access to areas a standard user normally wouldn't. It give you complete freedom to do what you want with your device, but remember, with great power comes great responsibility. Be careful or we'll be here in a couple of days walking you through a complete reflash of your phone.
As a location, it means the topmost directory in a volume. For example, Windows identifies drives with letters. C:\, D:\, F:\ etc. Your programs go in c:\program files\. The root location would be c:\. On a Linux/Android device your volumes are given specific mount points. The user files would be in /home/username/ with the root folder being "/" (note the difference here ... forward slash instead of backslash and no drive letter ... and my overuse of parentheses
).
When you put an SD card in your device Android gives it a specific mount point. You'd think it would be /sdcard and you would have been right three years ago, but as time went by and devices came equipped with 8+ gigs of internal storage, manufacturers mounted the internal user space at /sdcard (it was a good idea, BTW for many reasons, but it makes this confusing, even without all the parenthetical comments.). Where they mounted the external SD card was up to them. If I recall (I don't have my S2 with me, so I can't check. (It's been rooted and flashed with a custom rom anyway, so it wouldn't be the same as stock. -- WHOO HOO, DOUBLE PARENTHESES!)).
Here's what I think you want to do - copy the su.zip file to your external SD card and then put the card in your phone and flash the file to gain root. If that ain't it, get the newspaper and ignore everything that follows.
Put your SD card in your PC. It will assign it a drive letter like "G:". I'm going to use that for now, but you will use whatever drive letter really gets assigned. Copy the file to G:\. That's the root of your external SD card. Now, put the card back in your phone and boot into Android. You will now need a file manager like Astro or ES File explorer (free in the play store). Find your external SD card which most likely will be in /sdcard/external sd/ or /sdcard/sd-external/. Now copy the su.zip file to /sdcard which is what Android thinks is the root of your sd card.
You can continue from this point with a regular flash. Power off the phone, boot into recover and choose install from sd card. You should see su.zip listed. Install and then reboot. And if you did everything right and the gods are happy with that yak you tossed into the volcano (Don't leave that part out ... the root gods love fresh yak) you should be all rooted. Open your app drawer and see if SuperUser is there.
I hope that helped.
