When using the import from Excel Utility the "Use accout numbers" checkbox will enforce that you've assigned a column as account numbers on your import file, in order for accounts to be displayed. In addition if you check to "Use account numbers" any rows in your import sheet without an account number will get filtered out.
In the following example, I've checked the "Use account numbers" checkbox on the Import Option sheet.
Next I assign the columns on my import sheet, but fail to identify a column as containing account numbers.
When I advance to the Assign Accounts screen none of the rows show up from the import file.
The reason no accounts are showing is because the "Use account numbers" was checked, however no column on the import screen was designated as including account numbers. If you have checked the use account numbers checkbox you must designate a column as containing account numbers, and all rows must have a value in that column.