1-While creating a campaign: When you activate Marvin for your campaign and use "No Revenue Optimization" on information step, you can set target CPA under Budget section of campaign creation process. Action type updates itself based on the optimization goal you selected for Marvin on the information step.
For instance, if you select "Website Leads" as optimization goal, you can set Target Cost per Lead.
2-Setting a tCPA for an existing campaign: Marvin bids should be activated in order to set a target CPA for a campaign. When you click on an active campaign in campaign management screen, you can activate Marvin bids on "Campaign Settings" section. After the activation, you can click on "Manage Bids & Budgets" button above and set a target CPA.
It might take some time (like 2 or 3 days) for Marvin to start hitting the CPA target depending on how strict it is.