For the SMS Notifications add-on to work, you need the Clickatell API ID. To get it, do the following:
Go to the Clickatell website and click Sign Up to create a new account. Check the link from your email to confirm the registration, then sign in.
Click SMS on the sidebar and choose Configure SMS
Set up your test phones. Click the Add test phones button.
Enter your test phone number in the opened window and click Add phone.
Now configure the SMS notifications. Press the Create New Integration button.
In a new window, on the Details tab, enter the name and description of the key. In the API Type field set HTTP.
On the Features tab, in the Messaging Type section choose One-way messaging. You can choose any option in the Delivery Type field.
On the Settings tab, turn Enable message parts on. This allows you to send messages longer than the normal character limit. Leave the rest of the options turned off.
You do not need to make any changes on the following tabs. Finish your configuration by pressing the Finish button.
Now you have the API key. Click it to see the details.
In the new window, copy the API key.
Open the settings of the SMS notifications add-on and paste the key value into the API ID field.
Click Save.