There are two possibilities here:
1. The user did activate their old system (where the books were downloaded) with Reader (or Acrobat 7) using an Adobe ID or Microsoft Passport.
2. The user did not activate their old system
First case:
In order to activate your new system, you have to have Reader 7 installed. It is the piece that actually does the activation. So here's what you need to do:
Get Reader 7 here http://www.adobe.com/products/acrobat/readstep2_allversions.html.
Log into your computer (assuming it actually has a login but they do need to be in the account that you used when you bought the ebooks)
Start Reader 7
Go to the site www.aractivate.adobe.com (or just go to File/Digital Editions/Activate Device...)
Login there with the same ID you used before.
Push the activate button
If you see a screen about Could not find Reader software blah blah choose Yes, my computer does have .. Blah blah.
Reader 7 should then handle the edn file sent down and tell you that your computer has been activated. If for some reason the EDN file isnt handled automatically, just tell the OS to have Reader 7 handle the file.
There will be a pause while Reader does its thing. Then you should be presented a dialog telling you that your computer is activated.
Second case:
If the user did not activate their old system, there are several possibilities:
The user no longer has access to the old system (disk reformatted, lost, left company, etc.).
o In this case, there is no real hope. The keys required to open the book were on the old system. Even we cannot retrieve them.
The user still has access to the old system In this case, they should do the following:
o Activate the old computer using the exact same procedure as outlined above for the new computer, using an Adobe ID (they could use a Microsoft Passport, but even Microsoft deprecates it now and they will need an Adobe ID for Digital Editions 1.5 anyway).
o Then proceed as indicated for the first case and activate their computer using the exact same ID
Both Cases:
We then recommend that you install Digital Editions 1.5 Beta, which will then convert your books to the enhanced DRM format which makes them portable.
After repeating the steps on your new Mac you should be able to open your books. Then upgrade to Digital Editions 1.5:
http://labs.adobe.com/technologies/digitaleditions/
More info about it is available here
http://blogs.adobe.com/digitaleditions/2008/02/digital_editions_15_beta_is_re.html
And here
http://blogs.adobe.com/digitaleditions/2008/02/digital_editions_and_content_p.html
This is a little involved but should work. Let me know how it goes.