I agree with hitest in post #4 where he recommended you install sbopkg. It makes installing slackbuild packages much easier. But here is a step-by-step set of commands with comments that you can do without having to install sbopkg.
Open a terminal window and do the following:
Go to your home directory and create a directory called openarena
$ cd ~
$ mkdir openarena
Go to your openarena directory and download the slackbuild tar file from slackbuilds.org.
$ cd openarena
$ wget http://slackbuilds.org/slackbuilds/14.1/games/openarena.tar.gz
Untar the slackbuild archive.
$ tar xvf openarena.tar.gz
Go to the openarea subdirectory created by the tar command and download the openarena zip file.
$ cd openarena
$ wget http://download.tuxfamily.net/openarena/rel/088/openarena-0.8.8.zip
Or you can copy it from your home folder where you previously downloaded it.
$ cp ~/openarena-0.8.8.zip .
Make sure the slackbuild script is executable.
$ chmod +x openarena.SlackBuild
Switch to root. Don't forget the dash after the su. Enter root password when prompted. This will change your current directory to root's home, so you'll have to change directory back to your slackbuild directory. Substitute your real home directory for /home/rita below.
$ su -
# cd /home/rita/openarena/openarena
Execute the slackbuild script.
Exit from root.
I see on the openarena page on slackbuilds.org that OpenAL is a prerequisite. So you'll need to first install OpenAL before you install openarena. Just follow the instructions above with suitable substitutions.