Being the Administrator, why can't I login as a user?

I took the ADM201 class, and we had access, I assume , to the enterprise ediditon. In that edition, I could go to "Manage Users" and select "login" for any user to test various elements.

Since, then we are now on the Professional edition, and I do not have that option. Is the login as any user not available in this edition? Or how can I activate?