You won't find any help here on how to use someone else's BlackBerry device. Call the carrier on the device to which it is branded and tell them. Remove the battery cover on the back and give them the PIN number on the white sticker.
You can also call your local police as well as the place you found it (mall, shopping center, cab company?) and turn it in so that the rightful owner might have the opportunity to get their device (and likely their life) back.
Please do the right thing, same as you would hope someone would do for you.