Are you refering to this?:
http://www.maxon.net/pages/solutions/archicad/archicad_e.html
This is talking about the Material Exchange plugin, which is different to the Maxonform plugin you seem to be talking about (it gives option to 'Send back to ArchiCAD')
The Material Exchanger is a 1-way plugin to map materials applied in Cinema to ArchiCAD materials. Using this you can simply export updated geometry out of ArchiCAD and the plugin does all the time-consuming work of re-applying the materials (near instantaneously). There is no functionality to send anything back to ArchiCAD.
As far as I am aware you cannot download this plugin from the Maxon site - only updates. You need the original plugin installer which came on the Cinema 4D Architecture Bundle CD.
Maxonform enables you to edit/create new elements within Cinema4D and send them back to ArchiCAD as a GDL object. It is activated from within ArchiCAD. There are two plugins you need to install - one for each application. It sounds like you have only installed the Cinema part.
Maxonform is not supported anywhere on the Maxon site - refer to the links above for Graphisofts instructions.
cheers,
Owen Sharp
Design Technology Manager
fjmt | francis-jones morehen thorp
iMac 27" i7 2.93Ghz | 32GB RAM | OS 10.10 | Since AC5