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Hi,

You need to login to your Student account on this page:
https://www.autodesk.com/education/free-software/autocad-for-mac

and look for the serial number on your account management page:
https://manage.autodesk.com/cep/

If you don't have Student account, you need to create it on the page I mention above:
https://www.autodesk.com/education/free-software/autocad-for-mac

After you will get your Student serial number you will be able to activate Trial software.