Thanks eddyk!
For the English version, the option to select is "Match Field".
To explain it in more detail, you need to tell the database which field in your import file matches the primary key in the table you are updating.
After you selected the file to import, you will see a list of fields from the import file along with a list of fields from the table you are importing data to.
Make sure the primary key from your import file corresponds to the primary key of the database table.
Then click on the field in your uploaded data that corresponds to the primary key. A menu will pop up listing sample values followed by a few options - Ignore Field, Ignore all Fields, Import all Fields, and Match Field.
Select "Match Field" from the menu that pops up. Once the two fields are matched up, the Import button will no longer be greyed out.