You must approve a custom object before you can use it. The process is slightly different for new custom objects and ones that you've edited.
Approve a New Custom Object
You've created a brand new custom object. Here's how to approve it.
1. In Admin, click Marketo Custom Objects and select an object that's in a Draft state.
2. Click the Custom Object Actions drop-down and select Approve Object.
A custom object used in a one-to-many structure must have at least one dedupe field, a link field, a linked object name, and a linked field name to be approved.
A custom object used in a many-to-many structure doesn't need a link field, linked object name, or a linked field name when you approve it (because they live in the intermediary object).
A custom object used as an intermediary object requires a link field, linked object name, and linked field name but doesn't require a dedupe field.
See Understanding Marketo Custom Objects for more information.
Approve an Edited Custom Object
After you edit an approved custom object, you must approve the draft to return the custom object to an Approved state.