Obj's should open without a plug-in. Exporting is different. With your scene open - file -import (check box)- file type- OBJ. When the window pops up select -Merge into selected namespaces and rename incoming objects that match. Select single object, you can always separate it in Maya, otherwise objects come in in random sets. You might want to check your outliner after because size can vary quite a bit in Maya if you can't see your object.
