Where do i find the Traktor Pro 3 that was included to Traktor Kontrol S2

You need to download and install Native Access. You also need a Native ID if you don't have already one.

After installing Native Access open it, log in with your Native ID. If your S2 is already registered to your account, just install Traktor Pro 3 with Native Access. Otherwise use Add Serial to register your S2 - use its hardware serial.

Some caveats:

1.) If your computer doesn't support Traktor Pro 3, Native Access won't offer it to install - even if your S2 is registered to your account.

2.) If you bought your S2 used (or is already registered to another old Native ID of yourself) you cannot register it. In that case you will need a license transfer ID from previous owner to do so or contact Native Instruments support with a proof of purchase and picture of the hardware serial on the device.

Following links may be useful to you:

Buying or Selling a Native Instruments Product Second-Hand

How to contact NI Support