Category importing is similar to product importing. Here is an example of a working import file with the minimum required fields - note that the 'url_path' field is used to determine how categories are arranged in the catalog. 

  is_active,name,url_path
  Yes,"Hats",hats.html
  Yes,"Top Hats",hats/top-hats.html
  Yes,"Wide Hats",hats/wide-hats.html
  Yes,"Silk Top Hats",hats/top-hats/silk.html
  Yes,"Goggles",goggles.html

This produces this layout:

  Default Category 
   |-> Hats 
   |   |-> Wide Hats 
   |   `-> Top Hats 
   |      `-> Silk Top Hats 
   `-> Goggles