[edited slightly to be clearer]
Hi Helveticus,
I'm not sure how you're doing this.

I do:
Code:
myusername$ crontab -e
[edit my crontab file, save (no need to pick a filename) and exit]
myusername$
Then that is it - my crontab is saved, and active.
However, if I want to debug a bit further, and see that the file is created, then, as root, I do this:
Code:
# ls -l /var/spool/cron/crontabs/
total 4
-rw------- 1 myusername crontab 1089 Aug 12 14:30 myusername
#
Basically, when you do 'crontab -e', it's a wrapper around the cron system - it makes sure your file is correct before being inserted into the crontab system proper. This means that you do not need to pick a filename, the system picks it automatically for you, and then does the right thing.
I hope this makes a little more sense - you don't need to worry about the internals of cron too much to be able to write a crontab.
You don't *need* to know, as a user, that cron stores all user crontabs in /var/spool/cron/crontabs, it's just some internal info I provided.
Good luck!
Sid