Help with cron settings?

[sc <sunkcost@pobox.com>]

(don't think this one went through.  Apologies if this is a repost)

I'm having troubles getting my backups automated.  I set up a file "test" 
in the cron.d directory with the settings...

5,10,15 * * * sunkcost /bin/tar -cf /home/sunkcost/test.tar 
/home/sunkcost/test.txt

I thought that cron was supposed to check through its crontab and related 
files every minute or so, but nothing happens.  I tried restarting cron 
manually, root as the user, and editing crontab directly, but no dice.

I'm probably missing something really obvious for what seems like a 
straightforward setup, but I'm stuck.  Can somebody give me some help 
here?

TIA,

--SC