1) Download ADB (Google it...)
2) Connect your phone via USB
3) ADB is a DOS like thing, use the command "ADB Devices", of course without quotes
4) If it lists back something, that means your phone is still being detected as a USB device.
5) Use command "ADB REBOOT DOWNLOAD", it should force the phone to reboot in Download mode.
Post here if it worked for you or not.