There are two ways to get to the Manage Pages interface for any community, which is where LAR import/export functionality lives. You can access it by going to the Welcome Menu on the page, and clicking "Manage Pages." If you use this path, you will be unable to import a LAR file, as the application considers you to be "in" that community, and can't overwrite the pages with the LAR you're trying to import. Instead, you'll want to access this page by going to Control Panel -> Communities. For the desired community, click Actions -> Manage Pages. Now you should be able to go to the export/import section and import your LAR.