Installing UpsellPlus adds the app to your Shopify admin and, depending on the surfaces you plan to use, one or more storefront components: a theme app block for product-page and cart-page placements, an app embed for the cart drawer, a post-purchase UI extension, and — for Shopify Plus stores specifically — checkout UI extensions. None of these do anything until you both enable them on the relevant page or in the Shopify Theme/Checkout Editor, and create an offer that targets that context — enabling the surface and creating the offer are two separate steps, and merchants new to the app often do one without the other.
An offer needs three things at minimum to show: a product (or a ShopBrain source, if you're not hand-picking one), at least one context from the Surfaces section, and no active Smart Rule blocking it. Everything else — discounting, display styling, translations, behavior mode — has a sensible default and can be left alone for a first offer.
Checkout and checkout-header/footer placements are the one surface with an extra requirement: Shopify Plus, plus activation inside the Shopify Checkout Editor itself, since Shopify treats checkout extensions as opt-in per store regardless of what's configured in UpsellPlus. If your first offer is a checkout offer and nothing appears, that Checkout Editor activation step — not the UpsellPlus configuration — is the most common thing missing.
For the click-by-click version of any of this, the Help Center's getting-started guide and onboarding walkthrough cover the actual admin UI; this page is about the concepts those steps assume.