Try each of the following steps until the issue is resolved:
    • Add the Address Import ID for each address to your import file
    • In Administration, click Import
    • Open the appropriate import parameters
    • Select the Fields tab
    • If importing only one address, ensure that the extension is set to 00 for all the address fields.  If importing multiple addresses, ensure that the extensions for each address match.  For example, for the first address, ensure that the address import, city, state, zip, etc. all have the same extension such as 00.  The second set of address fields should also have the same extension such as 01, and so on.
    For more information on importing addresses, review Knowledgebase article Address imports as an alternate address.