Cannot find EndNote serial number of previous version


I just changed computers (new version of macbook Air) and transfered all my applications, including EndNote X9. When launching the app, it asked me to enter the previous serial number which I couldn’t find (I purchased Endnote a long time ago and do not have the email with serial number etc anymore). So now my Endnote is on ‘trial’ version only. 

How can I solve this? I do not want to purchase another EndNote as I had one already.