From that message it appears Wavelab is looking for a different Voxengo plugin -
SPAN Plus. If you didn't download SPAN plus (or already have it installed), either download and install it or make sure it really was MSED you downloaded and not SPAN Plus.
I had downloaded MSED v2.4 a while back and just now installed it without a hitch in Samplitude 11 Pro. I noticed a new version, 2.5, is available so make sure that is the version you have and perhaps try the 32bit version rather than the 64. My version of Samplitude is only 32 bit while my Windows 8 is 64 so I can't run the 64bit MSED until I upgrade Samplitude 11 Pro to Pro X (aka v12) which has native 64bit support - 11 Pro only supports 32 bits. Perhaps Waveleb 6 has the same limitations.
< If you can't get the Voxengo free MSED tool to load/work you can try one from Kelly Industries mentioned in
this thread. I just checked and the download link is still good though it is no longer available directly from Kelly Industries main page. >