Are you looking for Samsung Galaxy S II USB drivers to use ADB, DDMS and/or other Android development tools?

1. For any OEM Android device you can find the links for OEM USB Drivers on official developer.android.com site.

From there this is the location I generated and used to get software (driver kit and Kies) and manual for Galaxy S II, ATT carrier (model name SGH-i777).

http://www.samsung.com/us/support/downloads/at-t/SGH-I777ZKAATT

2. Do not forget to enable USB Debugging <Menu(Apps)->Settings->Applications->Development->USB Debugging> on device. By default it is disabled and therefore your device is detected as MTP unit which you still can use if install Kies (PC Software for Mobile from Samsung).