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I have Civil 3D 2015. The Attach/Detach Object Data Dialog box does not display. It shows up in the command line but I need the dialog box. I have tried CMDDIA, it was set to 1. I changed it to zero, saved the drawing and then changed it back to 1, it didn't help. I did the same for FILEDIA and ATTDIA, just in case, but nothing worked. I even rebooted and tried it again. Thanks you ahead of time.
Have you tried performing a repair on C3D?
Mine was not working either but I did find a post from Autodesk on how to fix this.