This seems to be a known issue with WaveLab 6 montage - it does not want to accept multi-channel-capable plug-ins. This can be fixed by loading the plug-in into the master section of WaveLab and then enabling the "No Multi-Channel Operation" check-box in plug-in's global settings. This setting, however, applies to all Voxengo plug-ins, so if you work on both multi-channel and stereo material, obviously, this solution won't work for you.