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

Example (fullpath of the SO Library):
/home/john/Softimage/XSI_5.0/Addons/ms_xsiMetaballs_v2-0/Application/bin/nt-x86/ms_xsiMetaballs.so
