You can use this CD as a repository for slapt-get/gslapt. In order to
do that, you need to edit your /etc/slapt-get/slapt-getrc file. Edit it
with your favourite text editor (as user root) and comment every SOURCE
line in it, by putting a # in front of every line, for example this:

  SOURCE=http://download.salixos.org/i486/slackware-15.0/:OFFICIAL

should become:

  #SOURCE=http://download.salixos.org/i486/slackware-15.0/:OFFICIAL

then add a new SOURCE line that points to your mounted optical disc, for
example:

  SOURCE=file:///media/Salix_15.0/:OFFICIAL

You will need to update your slapt-get package database by running:

  slapt-get -u

and then you can use the CD as if it was a regular repository.