Ok, I see on www.slackware.com
that the latest release is 13.37 so I assume this is your version. Up untill version 12.2 (the latest I have used) I used to download packages form here
and install them from terminal.
# installpkg <packagename>.txz
I see in the Release Announcement that you can use slackpkg too, but I don't know how this works. Take a look at the man pages of both programs.
You should also take a look at Slackbook
There is a package management section.
Edit: You know that you can use k3b instead of xcdroast wich comes with KDE (in your other thread you mention using KDE, right?).