Please post the EXACT version of maya you are using.

The obj plugin has gone through several instantiations depending on the version of maya you have.

After autodesk took over they apparently decided to scuttle objexp to foist their own fbx format on everyone.

I currently am using maya 2009 unlimited and I have both the fbxexport and objexport mlls, but I can swear that some earlier versions just after autodesk took control the objexporter disappeared.