EudoraNicknameIAD is an Internet Address Detector Action to add an email address to your Eudora Nicknames file. As usual, you should install it by using the Install Action File option in the "Apple Data Detectors" Control Panel.
EudoraNicknameIAD requires the Dialog Director 0.7 OSAX.
Users of multiple Eudora settings files: If Eudora is not running when you envoke EudoraNicknameIAD, you must have a "Eudora Folder" (or an alias) inside your "System Folder", so that Eudora will get launched automatically with the correct settings file. Otherwise, Eudora must already be running with the appropriate settings file before envoking EudoraNicknameIAD.
Users of Eudora 5.1 (or later): For reasons that I don't quite understand, the (undocumented) internals of the AppleEvent interface to the Nicknames file has changed in this version. Please download this version of EudoraNicknameIAD instead.
There are few informative pages on the web about Internet Address Detectors. The most useful ones I've found are at emailman.com and the Data Detector Depot.