For login , first you have to click on "choose API Account" in the left menu.
Then select API account(Sandbox or Live).
After selecting account , click "Generate Token".You will redirect to a page which shows textbox for entering user credentials.
Carefully fill your user credentials and click "Try it out" button.
If credentials are correct, your access token will be generated and use this token for other API calls
If not, an error message will displayed.