I had the same problem. I use yosemite. Downgraded from 4.0 to 3.0. Same symptom.
Apparently the default in yosemite (compared to what I was used to in Leopard) is to HIDE the library folder. So I didn't think the path above existed.
The default view of the ~/Library folder is hidden. So while hidden you can't navigate to the activation.dat file in a finder window.
You can either "GO TO FOLDER ~/LIBRARY" or just make it visible permanently.
You do this by entering within Terminal the following command and press Return:
chflags nohidden ~/Library
Now you will see ~/Library. (~ is shorthand for your login username for the linux naive).
This resolved the immediate issue (of E_ACT_NOT_READY). But as seen in other postings, ADE 4.0 crashed after loading a acsm file. I downgraded to ADE 3.0 and life is good again. It all used to work with my old system running ADE 2.0 and leopard.