I tried doing that (as root) and got the following response:
root:# slapt-get --install wxgtk-2.8.12-i686-2cf.txz
Reading Package Lists...Done
No such package: wxgtk-2.8.12-i686-2cf.txz
No, it's not in the Vector repositories. It is a Slackware package. So, you would need to download the package, and then right-click on it within Thunar, and select "install package". Or, if you prefer the command line, download the package, and using the terminal, go to the directory that you've downloaded the package, and use the command "installpkg wxgtk-2.8.12-i686-2cf.txz".