Adding coupon code

Would you assist me in the method for offering a coupon option.

1. Getting the code recognized at the Cart/Checkout:
At Appearance / Widgets: I see the Plugin: Mobile Content Modal (Version 1.0) but I can’t figure how to use it. (I tried SAVE15%, alternately, to Text and Virtue:Image. But SAVE15% does not show up anywhere. Checkout says coupon does not exist.)

2. Getting a statement on each page telling visitor to add the code at Checkout:
At Appearance / Theme Options / Top Bar Settings, I tried adding text under Title & Description in the Icon section (and I Saved Changes) but it doesn’t show up anywhere on the website page.

Thank you very much.