Amazon sets the download manager version at 1.0.3 when you say you have it already installed in the dmusic_download_manager_enabled cookie.  We need to edit the cookie to 1.0.10 in order to be allowed to download the enhanced content AMZ entitlement files.

It should be noted that right now, libamz 0.1.0 does not really support the enhanced content so you should buy the normal MP3 albums unless the enhanced content album is cheaper for some reason.  But since I have purchased such a file, I will release a 0.2.0 release shortly which also downloads the other data.

You can edit the cookie with a Firefox extension called, ironically, “Edit Cookies“.  In Chrome, you can use the “Edit This Cookie” extension, but I don’t use Chrome very much so I haven’t tried it.