Hi & Welcome to Linux Questions.
Looking at the SlackBuilds page you will need to install these dependencies.
FreeCAD requires xerces-c, Coin, SoQt, Pivy, and OpenCASCADE
Install the dependencies in order and than install install FreeCAD.
If you want to install package A and it depends on B which depends on C, you have to install C before you can install B before you can install A.
After running the SlackBuilds script you should have a package with the SBo.tgz extention in your /tmp directory.
From the /tmp directory you should be able to use installpkg to install the first dependency.
<OR> you can use pkgtool to install the package.
The error says that the file is in the wrong format ".so" and it should be .tgz.
http://docs.slackware.com/slackware:..._off_slackware
http://www.slackbook.org/html/packag...utilities.html