You have to start out by creating a DEVCLASS with devtype=file.
That specifies the directory you want the file sent to.

Here's an example:
DEFINE DEVCLASS dbbkuptodisk   DEVTYPE=FILE FORMAT=DRIVE MAXCAPACITY=204800K
MOUNTLIMIT=1 DIRECTORY=/var/adsmpool SHARED=NO

"dbbkuptodisk" is a made up name, use anything you like.
MAXCAPACITY should be large enough to hold one DBBACKUP (although the DB
backup will work OK if TSM has to break it into pieces).

Then when you do your DB backup, it should look like this:

backup db type=full devclass=dbbkuptodisk scratch=yes

TSM will create, on the fly, a new file in the /var/adsmpool directory to
contain the db backup.
If the db backup is larger than maxcapacity, TSM will create a more file and
the db backup will span the two files.
You can find out what TSM calls the file by looking in the activity log or
in the volumehistory file.
(Or just looking in the directory....)

The older db backups go away when you run DELETE VOLHISTORY (same as if your
DB backups were going to tape).