To go to Creative Designer, you need to click on Assets > Product Catalogs first for using it in catalogs. Then you need to click on the "Manage" button of the product catalog you'd like to use.
You will then enable creative designer for whichever feed you'd like to work with by clicking on the slider next to "Open Creative Designer".
You'll be directed to this page when Creative Designer is opened:
You can either choose to upload a static image from your computer by clicking on the "Upload File" button or you can check the "Use Dynamic Image" option if you're running a dynamic campaign. Once you choose the image, you can update the canvas area by choosing the aspect ratio you'll be using. Shapes, extra images, texts, ratings, icons and weather overlay can be added by clicking on the related button on the left side.
In order to use Creative Designer in static images, you need go to Creatives in the campaign you'd like to use it in. You need to click on "Edit" in Creatives in the already created campaign, or you should go to Creatives step while creating the campaign. In the creative that you're creating, when you go to the Image section, you'll see the Design a Creative button.
When you click on the button, you'll be redirected to the Creative Designer page and you'll be able to make changes to the creative you uploaded to the Creative Designer.
When you click on the "Apply" button the changes will be saved and you'll be seeing the creative you edited on CD as your image. Please note that you can only save them as templates and can't download the edited images.