Mh, this is not the right forum section for sure, as your question is not related to aMule, but which one would be the appropriate one?
OK, anyway: Usually the password is provided (or is the url of) on a link page. Check the file name and the file comments for details.
e.g. File: themostevilthingkryeverdid-by_www.amule.org.rar
Now you should first try the page's url - in this case
http://www.amule.org - as a password. If this doesn't work some variations like amule.org.
If this didn't work - let's pay the page a visit and search for the file, Usually we'll find all needed information here.
If there are no more information in the title, checking the file comments and opening the file (to see an overview of the files in the archive should the header not be crypted) brought you no information - well, it's the easiest to forget about it.
Why: Linkpages try to protect their files. Imagine someone releases the file NextReleaseWillBeSoon.rar at aMule.org and now some evil people at abcmule.org post the link at their pages claiming that it is theirs. As the people at amule.org don't want this, they decide to use a password, so that everyone who uses the file knows that it's a file released by amule.org (as he typed this password) and not by abcmule.org.
You may argue about the absurdity of these thoughts, but that's how it's done.