You will get this error if you have not built and installed the assembly that contains the custom STSADM command code. From the exception trace, it appears that STSADM.EXE is recognizing the command, but is unable to load the code which implements the command.
Remember, that you have to build the assembly and you also have to install the assembly into the GAC so that STSADM can find it.
Hope this helps,
John F. Holliday, Author, MOSS MVP