You can avoid all the hassles of manually installing Evolution on Slack by installing Dropline instead.
http://www.dropline.net/gnome/ Dropline is a large collection of frequently used packages, and it includes Evolution and all the supporting packages that Evolution needs. Installation of Dropline is fast and easy and I'd highly recommend it. (Note that Dropline will set your window manager to Gnome, but you can easily set it back to KDE by running xwmconfig.) Good luck with it and welcome to LQ as well as to the amazing world of Slack. -- J.W.