I found a solution to this problem. By looking up the CLSID for the class and using CREATEOBJECTEX(), I was able to work around this error.

New test code, (though I could have done this more elegantly)

&& MlReader {14AE4119-AB4D-449D-B41A-6278EDEAFC84}
&& MlWriter {20315AFA-7C65-41BF-98F0-A16DAB307D7E}
oMetalib = createobjectex("{20315AFA-7C65-41BF-98F0-A16DAB307D7E}","","")
cFolder = [All Indexes]
cDir = ADDBS("c:\metastock\"+ALLTRIM(cFolder))
oMetalib.CreateDirectory(cDir)
oMetalib.AppendSecurity([MSFT],[Microsoft],1)
oMetaLib.AppendDataRec(20010721, 0, 123, 200, 100, 123, 3221, 0)
oMetaLib.CloseDirectory()

>I am using VFP v 9.0 sp1 and I can instantiate a metalib object but referencing a method of the object generates the following error:
>
>"OLE error code 0x8002801d: Library not registered."
>
>I have registered the library with regsvr32 and this does not fix the problem. You can see it in the object browser just fine.
>
>code snippet
>

>oMetalib = CREATEOBJECT("Metalib.MLWriter") && works fine and gives a reference
>cFolder = [All Indexes]
>cDir = ADDBS("c:\metastock\"+ALLTRIM(cFolder))
>oMetalib.CreateDirectory(cDir)    && this method call produces the error
>
>