No product key on ms word app can't load it onto Macintosh desktop

We have recently purchased the new office 365 subscription and downloaded it onto 2 iPads. We have received a receipt for the purchase from the App Store. The app states we can also load it onto our desktop Macintosh. When trying to do this we can't locate it a s a purchased app in the App Store.

When we purchased the app initially we tried to use the email address attached to our iTunes account however it would not allow this and gave a message that this email address was reserved and to choose another email address. We then used a gmail address, it worked. We have spoken to Microsoft this morning and they said we needed a product key to enter when installing on our desktop, which we have never received. Microsoft support said they have experienced similar cases where people have downloaded this product from the iTunes Store.

Please help.