Hi Tamtam,
generally speaking it is not recommended to install on Slackware a package intended for another distribution, noticeably because the packager of the rpm package (in this case) assume the presence of other packages as dependencies, of whom probably some won't be met, either because the corresponding application or library is not installed in Slackware or is present but with a not adequate version.
That said a little googling shows that a very few people where able to successfully install mysql-workbench on Slackware 13.1 the regular way (that is to say, building a package with a SlackBuild). That had to do some patching and install some gnome packages in addition.
I would recommend using another tool instead, some are available on slackbuilds.org, see
this page. I use phpmyadmin sometimes and find it handy.
