First of all, to be able to access the managed accounts admin console, you will need to be a Workato Consulting Partner. To find out more about our Workato Consulting Partners Program, you can check out this link here.
There are two ways that you can manage your client's accounts:
1.) Using the Team feature. This feature enables multiple users (Consulting Partners) to work on the same Workato account and collaborate on recipes in a shared workspace. If the customer doesn't have this feature, consider upgrading the current plan.
For more reference in using Team: Teams & Collaboration
For the second way, this applies to users who already have an existing account that you would like to manage. Have your client go to this link to access their account settings, and enter your consultant account email under the "Account administrator" field.
Once that's done, your clients account should show up in the managed users section!