The objectives have recently been updated and the secret page URLs changed - to keep them more secret!
The link to secret page no.1 can now be found by going to the creation space page - you'll find the link just above the timeline.
You need to click on it to complete achievement/objective number 45.
However, you can only ever visit it once - when you are completing the achievement. If you visit it before you have unlocked this particular achievement, or try to visit it again afterwards the link won't be there!