To enable Pardot Engagement studio in your Pardot org. please follow these steps:
1. If you are in new Pardot UI interface, please switch to old Pardot Style by clicking on the gear icon in the top right corner and clicking on "Disable New Pardot Style".
2. Now go to your Pardot Dashboard and click the "Get Started Now" button.
3. Go through the slides and click on "Complete Activation".
4. When that is done, you will see that 'Engagement Studio' option will appear in your dropdown menu under the Marketing tab.
I hope this was helpful.