MicrosoftExcel, by default will strip away any leading zeros from your zip code field, as soon as you open up your import files. So, in order to get them back, you need to tell MS Excel that we are wishing to hold zip code data in this field and to make all characters display, even leading zeros.
Right click on the letter of the column, at the very top, just above row 1. On the sub-menu, choose 'Format Cells'.
Example: Right Click on the letter 'A'....
Then click on 'Format Cells' when the context menu appears.
In the list of Categories on the left, choose 'Special'. Then choose 'Zip Code' in the list on the right. Click OK.
This will force MS Excel to retain leading zero characters in your data. If you save the file as an XLS or XLSX file, then it will continue to retain your data properly. If you save as a CSV file, then reopen the CSV file inside of Excel, the changes are lost.