Well, the message is clear enough, I think.
If you want to use this option, you should install another print spooler than CUPS, because CUPS doesn't include it
For instance you could use LPRng instead of CUPS. It is available for Ubuntu 12.04 as you can see here
Or you could simply use CUPS' web interface to start your printer (much easier, in my opinion). Just fire up any browser and go to address:
PS lprng is not included in recent versions of Slackware and you should not install packages intended for another distribution anyway. Furthermore you wouldn't be able to in case of Slackware that uses a different package format.