Only a limited number of Windows Phone users have been allowed to download and install the WhatsApp beta. However, there is some talk that the stability of the app means that it will soon be offered to all Windows Phone users.
Right now though, you can install the public version of WhatsApp, which is a free download from the Windows Store. The service is free for the first year and 99 cents a year thereafter. To add the messaging app to your phone, simply click on the sourcelink.
Update to WhatsApp beta for Windows Phone adds icon for Document attach button
1. Document attach button prior to update
2. Document attach button following update
source: WhatsApp (Windows) via MSPoweruser