Found the issue, but firstly I must say that It was giving a boolean true /"Operation Suceeded" Value.
This is a bit of a funny one, turns out you need to add the Attribute to an element, in my case I add the attribute to the first element in the Dimension if one exists. Stikes me as odd, but I think I understand why, it seams you can create Attributes for any object type in the system not just elements, it probably just adds it to the }objects (e.g. }cubes) dimension as an alias or something...
Thanks for your time.
I'm releasing what Im working on (Yet another TM1 Documentor) for free at
http://www.daptech.com.au as an Downloadable Application Template for Daptech's product (The Application Daemon) soon to be launched. The Application Daemon is a rapid BI web application development tool.
edit: If only I had seen your post 15 mins ago - would have sorted me out...