So, to go to the architecture dashboard you must go to the lifetime first of the environment first if you are registering for the first time.
Then on the environment click on the plugins tab and you will see the architecture dashboard option. Once you click you will see another button saying Architecture Dashboard, click on it and voila you will be able to login using your credentials. Use the OutSystems credentials.
The link that @Vinod Patidar shared shoes that, try to find this block of content.

I am just sharing this because I also had some struggles during my first attempts to access the architecture dashboard. And all the first times I needed to go to the lifetime and from there I could go to the architecture dashboard, that's my experience.
Regards,
Márcio