I got the exact same message, and if you go into your security iconinyour settings, you can chose to receive downloads from any source (not just Apple apps or identified authors). But I'm afraid that that might not fix your problem. I changed that, downloaded ADE 2.0, migrated my older files (ebooks) and then got the error message from Apple that this was not a supported application. Sigh. I'm hoping that I can find another app or application to read my ebooks (there are only a few that I've had to use ADE for so far, thank goodness). Oh, I was using an ancient G4 running Leopard, and the problems popped up tonight with my new (wheeee!) Air running Mountain Lion.
Ok, you may have read the posts that I have now, but just in case... apparently ADE is not yet compatible with Mountain Lion, so we're stuck for a while unless we can go either to the publisher, or seller and request suggestions for how to read our purchases. Luckily for me most of my affected books are fiction, and not for work/school. I imagine that your Elsevier books are as nearly expensive as my Elsivier hardcovers - nice textbooks/reference materials.
YES! You are right, hjnazen8. Go to Settings > Security > click padlock to unlock it > choose Allow programs from any source. I have Polish version of Mountain Lion, so there could be different words in the pathway.
After that I was able to run installer DigitalEditions2.0.pkg downloaded from Adobe.com. It found my previous version of ADE 1.7.2, transferred my e-books to the new version and even suggested uninstalling old program, which I agreed. At the end there was a warning that the program can not run in MacOS, but I ignored it. Everything works OK. PS I just wonder why a person responsible for the program in Adobe could not help me and wasted my time.
And ... Good luck with your new MacBook Air!