After installation and activation, the plugin will automatically calculate product Upsells for you. Just go to Upsells > Products > Recalculate All Upsells. This will enable Upsells on autopilot throughout your whole website.
You can go to Upsells > Settings to fine-tune your strategy then Recalculate again if needed. Take note that for large servers, calculating upsells can consume a lot of resources, in that case, reduce the batch size to avoid time-out errors. You can do this by going to Upsells > Settings > General Settings > Batch Size.