copy the "nt-x86-64" folder in <siUserPath>\Softimage\XSI_5.0\Addons\ms_xsiMetaballs_v2-0\Application\bin\

Example (fullpath of the x64 DLL):
D:\users\John\Softimage\XSI_5.0\Addons\ms_xsiMetaballs_v2-0\Application\bin\nt-x86-64\ms_xsiMetaballs.dll
