If you are coming from the Plugins screen, you need to click on the Activate button on the plugin tile labeled Staff Scheduling and Time Management.
If you are coming from the Manage Account section, you need to navigate to Choose My Subscription section, on the Purchase Subscription screen, and put a tick in the activation checkbox to mark the plugin as Active.
In either case, the application lets you review your order summary before you click on the Confirm & Pay button in the Cart Detail section to process payment.