To create the import file:
(1) Create an Excel file with three columns: ItemCode,Description,Price
(2) Fill in the rows with your data.
(3) Click on the round menu button -> Save As.
Save as type: CSV (comma delimited).
Then choose a name and save it normally.
(4) As you know, the import function is under the Item List dialog.
It's best to enter a few test transactions after you've imported the data, to make sure the performance is within your expectations.