Thank you for your help -- I downloaded vmware-vdiskmanager for mac from you link and launches fine on the mac. Now unfortunately I'm running in another problem when trying to shrink the volume:
Failed to shrink the disk 'filename.vmdk' : Disk encoding error (0x3d00003ebd).
Not sure if that's because the vmdks were created using a vmware product on windows and somehow the mac version of vdiskmanager doesn't like them. I might resolve to copy over the vmdks to a windows box and shrink them there.
- Adriano