Some ereaders (eg Sony) require a driver before they'll be seen by ADE.
The Sony driver is installed along with Sony Reader software; no need to run the Sony Reader software but you do need the driver.
Maybe there is an equivalent driver needed for your reader?
Are you sure your eReader supports Adobe DRM .epub files? What kind of ereader is it?
ADE2.0 has issues with some ereaders.
You could try installing v1.7.2 instead of (or even as well) ADE2.0.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
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