You are here: Control Panel > Design Manager.
To import a design you have created start by clicking the "Import Design" Button in the top right of the screen (See Image Below).
You will then recieve a popup (See Image Below) which will allow you to upload your custom design, your design must be compressed into a ".zip" to be able to upload it. Note: You can import previously exported designs. When you have found your design you can then click the "Import Design" button, once the import has completed you will recieve a notification that you can now access your imported design under "Inactive Templates" in the Design Manager.