I bought a new MacBook Pro in 2012 and was stolen on a business trip in 2013.I have been searching for the serial number for years and now have found it. The people that stole it from me have caused me havoc for years.. How do I catch them now

You might not like Viking's response, but it's probably on point.

Unless you have hard, physical evidence that specifically identifies the individuals in question and proof of widespread criminal activity, most law enforcement agencies are not going to spend much time on a 5-year old case of laptop theft.

If you want to pursue it, and you're in the US, nationwide and/or international organized crime like this would be handled by the FBI, so reach out to your local FBI field office for assistance.
If the thieve in question are local kids, then your local police station would be the way to go.

so the question is , who can assist me in tracing the computer if I have the Serial number and can prove to be the original purchaser?

You cannot track the device by serial number. The serial number is only valuable to identify a device as yours if recovered by law enforcement. So, unless you have concrete evidence for law enforcement to use to recover the device from somewhere, then there is nothing that can be done.

This may be an obsession with you, but it won't be with law enforcement, or Apple five years after the fact. Let it go, and learn from the experience.

You're welcome.