I had major problems getting a solution to this.
Imported Idml files were imported ok but I couldn't get any tweaks done. None of the layers were editable.
But I found out that if you right click on the original layer the pull down menu changes, giving you the edit options. Edit Detached was the one I was looking for.
Now everything works and the fine tuning needed after import is a breeze.
There should be a tutorial on this. Many gnashed teeth, much wailing and lots of time got me this solution.