how can i install Adobe Digital Editions?

You can download the latest (most unreliable) version 2.0.1 from http://www.adobe.com/uk/products/digital-editions/download.html

For a more reliable version

There are lots of bugs in ADE2.0.  Try replacing ADE2.0 with the older but more reliable v1.7.2.

(You can have them both installed at once if you like.)

Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.

http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editions.html

The forum software is sometimes corrupting the link above.  There shouldn't be a blank in 'editio ns.html'.  The following redirects to the same page: http://tinyurl.com/diged172