Take a look at the PowerShell command Add-AzureDataDisk
. This should be what you're looking for, as you can specify media location of the uploaded vhd.
Alternatively, in the portal, go to Virtual Machines and navigate to the Disks tab, where you can create a new disk:
At this point, you can navigate to your uploaded vhd:
After this is done, the new disk should become available for you to add to a Virtual Machine.