After quite a bit of frustration, I have noticed these behaviors for my installation:
(Inventor Product Design Premium 2012, Win7 Pro 64 bit)
1. When opening a Fusion DWG file in Inventor, there must be a file open in Inventor in the background for the Application Icon flyout "Open Fusion DWG" option to appear - see attached showing same flyout with and without open background file. (Fusion Option.jpg, No Fusion Option.jpg respectively)
2. Whether there is a background file open or not, I cannot get Change Manager to launch using the standard Open... Autodesk Fusion File (DWG) dialogue option. I get message similar to "Can''t find solid body" or it opens with Fusion created edits but no Change Manager. (see Fusion Open.jpg). Change Manager will only launch when I use the Application Icon flyout "Open Fusion DWG" option.
The Inventor Fusion Change Manager and Fusion Freeform Edit Launcher add ins are loaded and set to Automatic in the Add In Manager.