moving from fusion to virtualbox

[Whoolly]

So I used Fusion's ovf export tool and want to move my VMs into VirtualBox. However, neither the OVA file will load during an import, nor can I just attach the vmdk.
When I try just the vmdk I get the following:

Failed to open the hard disk file

Could not get the storage format of the medium

(VERR_NOT_SUPPORTED)

Any ideas?