You can access the test functionality in a number of ways:
In the "Create promotion" wizard
From the Home page, click "Create promotion"
Go through the steps in the wizard
Click the Test button at the end of the "Create promotion" wizard
From the "Edit promotions" grid
From the Home page, click "Edit promotions".
Click one of the Test buttons in the grid of promotions to test the associated promotion
From the "Edit promotion" screen
From the Home page, click "Edit promotions". Then click the Edit button for the promotion you want to test.
Make changes to your promotion as necessary
In the "Edit promotion" screen, click the Test button on the top right.