Since the last Amazon changes, all keyword fields are weighed equally by Amazon's ranking algorithm so you can choose any order.  You can add them to:

• Seller Central Backend Searchterms - 250 Bytes
  -Every word only once
  -No special characters
  -Capitalization and order irrelevant
• Title - 200 Characters (inlcuding spaces)
• Backend Searchterms - 249 Bytes
• Bulletpoints - 1000 Bytes (around 2 rows per bulletpoint)
• Description - 2000 Bytes
• Enhanded Brand Content (EBC) - Not Indexed (products cannot be found via the keywords in EBC)