To map an already existing category to a Jet category, go to Catalog > Manage Categories. Select the required category that needs to be mapped (should be a non-jet category) and click on the "Jet Category Mapping" tab, where you need to select the required category from the dropdown, then click on Save Category button. Now go to Catalog > Attributes > Manage Attributes > Edit the required attribute > Jet Attribute Tab. Enter the “Attribute Id” of the Jet Attribute that has to be mapped with the Magento Attribute in the Jet Attribute Id field. Now click on the Validate button. When you map your Magento category, all products of the mapped category get listed in Manage Product grid of Jet.