Did you try to create the report on the UI? We generally recommend to create the report on the web UI first .Once the UI report is successful, you can proceed and recreate the same report via the API. If it is only the creatives that you want in your report, you can use the following report break down:
Dimension.CREATIVE_ID
Dimension.CREATIVE_NAME
Column.AD_SERVER_IMPRESSIONS
Column.AD_SERVER_CLICKS
Here is a complete list of report query parameters that you might want to check.
Please let us know if you've further question on this.
Thanks,
Tesfamichael Hailegeorgis, DFP API Team