If you are unable to successfully Activate DesignShop v11 and launch it in Sizer level, it will prompt you to always run as Sizer.
If you answer Yes to that dialogue box, you will no longer be prompted to Activate DesignShop v11 and it will always run at the Sizer level.
To make DesignShop v11 prompt for Activation again, click on Tools - Purchase DesignShop v11.
That will launch a Contact Melco page in your web browser which you can close. Next time you launch DesignShop v11 you will again be prompted for Activation.