You are currently not logged in. To be able to save designs you need to be logged in. You will be redirected to the login page after clicking on Save design. After logging in / account creation you will be redirected back to your current design so that you can continue working on your current design.
You need to provide a display name for your saved design. This name is shown on the Saved designs page so that you can easily find it on future visits.
After saving, your saved design can be found under Saved designs on your account page.