You don't.
If it is a dedicated eReader, ADE should see it when the eReader is plugged in by USB.
(BUT, see note below ~~~)
You can then authorize the eReader and copy books.
If it is a tablet eReader, you need to install an app such as Bluefire to the eReader.
~~~
There are lots of bugs in ADE2.0. One is that it fails to see some eReaders. If that is your issue ...
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-editio ns.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