If you try to load a plug-in that is compiled with a version of the Softimage.XSIOM assembly that does not match your version of Softimage, you will see an error message similar to this:
ERROR: Softimage failed to load a .NET plug-in. This error happens because the plug-in may have been compiled with a different version of Softimage. Recompile the plug-in with the current version of Softimage to fix the problem. <Plug-in: C:\Users\YourUserName\Softimage\2011-SP\Application\Plugins\nt-x86-64\PluginName.dll > <Current Softimage version:2011-SP.5.0>
Check to see what version of the Softimage assemblies are registered in the GAC (see To find out whether the Softimage assemblies are installed).
Recompile the plug-in using the version matching one of the versions registered in the GAC.