OK, I could install the software (the screenlets' framework and associated applications) after having installed a bunch of dependencies'(*).
However the upstream repo is dead (clicking Get more Screenlets in screenlets-manager leads to an unrelated website), so now one can only get the individual screenlets from
Screenlets PPA. Sorry, too much work for me to convert all these packages to the Slackware format, additionally there can be security issues doing so.
So as far as I am in concern you won't get these screenlets in Slackware
(*) list of dependencies, that I got from Salix repos:
libgnomeprint-2.18.8-x86_64-1salix
totem-pl-parser-3.4.5-x86_64-1salix
libgnomecups-0.2.3-x86_64-1salix
gnome-media-2.32.0-x86_64-1salix
BeautifulSoup3-3.2.1-x86_64-3tjb
libgnomeprintui-2.18.6-x86_64-1salix
gnome-python2-gconf-2.28.1-x86_64-1salix
lua-5.1.5-x86_64-1salix
gnome-python-desktop-2.32.0-x86_64-1salix
libquvi-0.4.1-x86_64-2salix
Additionally this is on Slint (Slackware derivative), presumably more dependencies are needed in genuine Slackware version 14.2.
For your information, the packages tagged salix come from
http://slackware.uk/salix/x86_64/extra-14.2/ (built using stuff borrowed to
https://slackbuilds.org/) andBeautifulSoup3-3.2.1-x86_64-3tjb come from
http://slackware.uk/salix/x86_64/14.2/
PS See also
this old thread.