You need Clickatell registration data (user name, password and API ID) to adjust the Clickatell communication for Alerts sending. Register on www.clickatell.com.
Then register for the HTTP product. Go to the HTTP API page and click on the grey button near the top right hand corner of the page which reads 'Register for Free'.
Now fill in your registration details on the registration form and press "submit".
The 3 bits of information that you will need to send an SMS will be your "username", your "password", and your API_ID. Your API_ID gets issued immediately upon confirmation of registration.