To import assers into Explorer click on File (located in the top left of the user interface), than select from the drop down menu Import.
After selecting Import, use the Import menu to navigate to the location of the asset you want to import. Select the asset and click on the import button (located on the bottom right corner of the import menu)
Once your asset is imported it will populate in the asserts tab.
Now Click on the New button (located at the top left of the user interface) From the New menu select the asset and click create.
Now your asset will populate in the 3D viewport