Hi Vicki,
Office mobile for iPhone is only available in the US iTunes Store at present. Based on our official information, the international launch date is 18th June, 2013.
If your Apple ID is signed up in US, please download it via the following link.
https://itunes.apple.com/us/app/office-mobile-for-office-365/id541164041?mt=8
If your Apple ID is signed up other than in US, please wait for the international launch. Thanks for your patient.
Regards,
August Sun