The OCR option is available under the Tools menu. Wen you select it, a setup dialog appears, where you can setup the OCR operation and decide how the results are to be used.
At the top left is the thumbnail pane. Here you can browse the document's pages. Click a thumbnail to select the page to be shown in the preview canvas. The preview canvas shows pages as they will be presented to the OCR engine. Depending on the quality and characteristics of the original page, you may need to adjust some settings to achieve optimal results.