The Gingerbreak.apk should have downloaded to the download folder on the sd card. You cannot download from a PC. It has to be downloaded from a browser on the phone.
If you have the stock 2GB sd card just move some things over to a folder on the computer until your done with the rooting process. Leave the Android, .android_secure, LOST.DIR, and download folders.
If you get a virus warning, disregard. You might have to delete the virus protection from the phone during download and install.
Use Andro Zip, Root Browser, or ES File Explorer to move Gingerbreak.apk to the root of the sd card (not in any folder, but in the same directory as the folders- /sdcard).
I used Dolphin Browser a couple of years ago to download the .apk because the stock browser was giving me fits.
Make sure you got the Gingerbreak-v1.20.apk from the second post here: [App] [26.04.2011][v1.2] GingerBreak APK (root for GingerBread) - xda-developers
Go to settings/applications. Make sure you check the allow from unknown sources before you download the .apk. Then go to the development option and make sure that the Debugging option is checked.
Once you have moved the .apk to the root of the sd card, then with a file manager just tap, or long press (depends on the file manager) the Gingerbreak.apk to bring up the install option. Once it has installed, select done.
Go to your app drawer and select the Gingerbreak application.
It it doesn't auto reboot the phone after 12 minutes, pull battery and try again. Could take up to as many as 10 tries (usually roots before 5th try though).
Once you are rooted then you need to make a backup of your Stock ROM from a custom recovery. This is very, very important.
For more info on this go here: http://androidforums.com/optimus-v-all-things-root/697813-how-install-flash_image-flash-recovery-rom-theme.html
If unsure of anything just ask. I would be happy to answer your questions.