To add a custom object tab, go to App Setup-> Create->Tabs and click on New button under the Custom Objects Tabs section.
Choose the custom object for which you want to create a tab from the picklist and then choose the tab style (image) for the tab, then click Next.
Choose for which profiles the tab can be visible. If the object you have chosen can contain fragile data it is better to set the visibility only for System Administrators as Default Off and set Tab Hidden for all other profiles.
Choose to which app the tab should be added, in this case you have to untick all tabs and choose only Pre.empt.
Then click Save. The tab should be available now for you on All Tabs list.