Authentication is described in the getting started guide, along with some detailed discussions on this forum. Do a search on the forum for “authentication”, there is a whole heap of detailed help.
I have noticed that the API Explorer does not show all of the request headers that are sent, one of which is the Authentication header, which would perhaps have been helpful. I will have that looked at.
Here are some key topic posts that should help you authenticate correctly.
I would recommend posting any follow-up questions to the following post: