Granted the thread is old, but the problem still floats around.
For those who still have this issue of finding their AuthUserFile absolute path to their web space (because your web hosting site is unable to), you can make a simple php file and run it from your site to find the answer.
Use notepad and make a file called
myroot.php (or any filename with the .php extension)
copy the following code into the file
--------------------------------------------
<html>
<head>
<title>Getting your AuthUserFile root location</title>
</head>
<?php
echo “<h1>Your website root location is --> “;
echo $_SERVER[‘DOCUMENT_ROOT’];
echo “ <--</h1>”;
?>
<body>
</body>
</html>
---------------------------------------------
Remember to save, and then upload it to your webspace, and run it from a browser on your webspace and copy the path to somewhere safe (and then remember to get rid of the file when you are done using it so a hacker cannot find your AuthUserFile absolute path for your web space).
It'll display the absolute path where your publically accessible webpages are kept, and more often your FTP can upload to one level up (which is a good spot for keeping password files, or making a folder from that level where the public cannot access, but since the AuthUserFile specifies an absolute path on the hard drive, you can choose a folder that you can only get to with an FTP application).
I've had a few ISP that had no clue where the site hosting was being done, much less the absolute path for my webspace, so I setup a little page for people wanting to try using htaccess.
http://home.myfairpoint.net/vze2xgg2...s/htaccess.htm
It provides simple examples which can often be the best to get people started, especially taking the time to explain how and why.
Before you post, read our: 
Can AuthUserFile in .htaccess use a relative path?
