You can see which sites are Credit sites by viewing the Supported Sites report. Upon launch of the system, only Bookmark sites are supported. New credit sites will be added to all tools in due course.
From the tools page, click the 'Submit New' button as normal and you will now see a new section on the job submission form. The list of available credits sites is displayed in a grid with the credit cost for each one in parentheses. Each site has a checkbox which you click to select the site. Below the grid you will also see the total of the credit sites selected so far and your remaining balance.
Click the check boxes to select sites; the totals will be updated as you do so and you will be unable to select sites that would take you over your credit balance. So if you have 3 credits remaining, you would not be able to select a PR4 site.
Everything else on the form is as before. Click the 'Preview & Submit' button to see a preview of the job and then hit submit.
Assuming your submission was successful, you can click the 'Details' link to the left of your new job and view the submission schedule. You will see a new column in this screen that shows the credits charged. You can also generate a credits report from the members screen which will show you the transactions for the credits spent on the job.