That's really odd.  It looks you have a permissions problem with your home directory. On startup, IDLE attempts to create the directory .idlerc in your home directory, /Users/ben, if it doesn't exist already.  If for some reason the directory creation fails, IDLE aborts.  Interestingly, if the directory exists but IDLE lacks write permission to create files in it, it does not abort but posts a warning message in a window.  Perhaps it could be a little more consistent about that.  But still, this appears to be avery unusual situation.  I can't think of any reason why IDLE would be unable to create a directory unless you have some security system installed or some unusual access control list setting.  The most likely reason is just a plain old permission problem on your home directory.  Try this in a terminal session:

cd ~
ls -lde ~

You should see something similar to this:
drwxr-xr-x+ 38 nad  staff  2992 Apr 28 15:26 /Users/nad/
 0: group:everyone deny delete

if the permissions string is missing the "w" ("dr-xr-x"), that means you do not have write permission to your home directory and can't create new directories there.  In that case, 

mkdir ~/.idlerc

should fail.  (This is essentially what IDLE is trying to do.)

If you are missing write permission on your home directory, you *should* be able to fix it by doing:

chmod u+w ~