Debbie,
When you download an ebook which is delivered through an Adobe Content Server, I believe the ebook reader (Digital Editions or Adobe Reader 6/7) needs to perform some form of authentication in order for the DRM to work. Before the advent of Digital Editions, you could choose to do a named activation of your Adobe Reader 7 or 6, but some form of anonymous activation always took place and still does, it seems.
By the way, here's a link to our free ebook for testing purposes. It's in Danish but if it works, any Adobe ebook should work:
http://services.publizon.dk/retailer/FreeEBook.aspx
Whether or not you can use Adobe Reader 7 depends on the kind of Mac you have. An Intel Mac (such as the Macbook) will not install Adobe Reader 7 according to Adobe.com, but a PowerPC Mac will install v7.0.9, as far as I know. Download it here: http://www.adobe.com/products/acrobat/readstep2_allversions.html.
The missing voucher folder problem is not something I'm familiar with. I've just tried deleting it; the folder recreates itself, but any ebooks on the machine cannot be opened and a download of an ebook subsequently failed. I restored the original folder and all was back to normal.
Digital Editions shows up in my Add/Remove Programs (I'm using WinXP). I've also noticed these related folders:
*C:\Documents and Settings\Your User Name\Application Data\Macromedia\Flash Player\www.macromedia.com\bin\digitaleditions2x0 - this is where the actual program is on a Windows PC
*Documents/My Digital Editions - where the ebook PDFs are located
*C:\Documents and Settings\Your User Name\Application Data\Adobe\Digital Editions\Voucher - the voucher folder
If you could post details of both the system that works as well as the one that doesn't work it would be great.
I have of course contacted Adobe about this but that doesn't mean we can't try to work it out ourselves while we wait for an official response :-)
Martin