nicomen is correct. I remembered that DoMethod has been moved from amiga.lib to intuition.library with OS4.0, but I missed that it was also renamed to IDoMethod.
On OS3.9 you have to include clib/alib_protos.h for the DoMethod prototype.
Sometimes you should look into the include files. If you don't find DoMethod in intuition_protos.h, you know that you missed another include file.
Bye,
Thomas