First, ensure you have reviewed and actioned all promotion errors before attempting to start the promotion.
There are three ways to start a promotion:

1. A promotion will automatically start overnight on the Start Date entered when the promotion was created.

a. If you need to adjust the Start date, highlight the promotion and click Change

The Promotion maintenance - Change window is displayed.

b. Adjust the Start Date and click Next.

The Processing window is displayed.

c. Click Cancel to return to the Promotion Enquiry - New Promotions window.

Or, 

2. It can be manually started.

a. Highlight the promotion and click Start and the promotional products will immediately scan at the promotional price.

c. Click Cancel to close the Promotions Enquiry window.

Or 

3. To start/stop multiple promotions at once:

Note: This process will Stop any promotions Current where the End Date is today or in the past. This process will Start any New promotions where the Start Date is today or in the past.

a. From the Promo Manager module, click the Functions menu and select 2. Auto Start/Stop Promotions.

The Auto Start/Stop Promotions window is displayed.b. Check the Continue checkbox and click Okay.

The report is displayed listing all promotions that were Stopped and Started.