To build SOM classes for use in REXX, you need to use the SOMobjects Developer Toolkit, Release 2.1 or later, or the Warp Toolkit. You must build the classes into a DLL that can be dynamically loaded, and the loading of the DLL should result in the building of all SOM classes within that DLL. The SOMobjects Users Guide explains how to do this. It is necessary because REXX calls the somFindClass on the SOMClassMgrObject to locate the SOM classes on import. The dllname modifier must be in your IDL file so that SOMClassMgrObject knows which DLL to load for the class.
You also need to define the class in the Interface Repository, so that REXX can get information about the class's methods, method parameters, and return types.