It is possible that when first asked to approve the PromoSimple Giveaway Tabs application you may have clicked on 'Skip' instead of 'Authorize'. This app must be authorized in order to communicate with Facebook and add your promotion to your page(s).
To change the permissions for the App you can log out of Facebook and then refresh the Publish tab within your PromoSimple account. You will see the 'Login w/ Facebook' button again and will be able to authorize the application so that it is able to access your pages, which is needed in order to add the tab to your Facebook page.
If, after logging in, you are not given the option by Facebook to approve the App, you will need to go to the URL below and remove the PromoSimple Giveaways Tab application from within Facebook. Then, go through the steps of adding it from within the Publish tab. https://www.facebook.com/settings?tab=applications