Below is a listing of how you should format your unit of measure in your import file.
On an item list import: Use the U/M Set List Name (in the screen shot below Each to Case/Doz)
For transactional imports the U/M Set needs to specified on the item in QuickBooks pre-import.
On a sales transaction import: Use the abbreviation (in the screen shot below ea)
On a purchase transaction import: Use the abbreviation (in the screen shot below cs)
If you are using a single unit of measure use the value that is the base unit name field on the Unit of Measure set in QuickBooks.