My computer is a 2007 Macbook Core 2 Duo. Meet all the specifications and running Ubuntu 14.04. Anyhow it won't install. I get this error message:

Preparing to unpack .../gog_star_wars_dark_forces_1.0.0.9(1).deb ...
error: XDG_RUNTIME_DIR not set in the environment.

(zenity:4576): Gtk-WARNING **: cannot open display:
dpkg: error processing archive /home/julian/Downloads/gog_star_wars_dark_forces_1.0.0.9(1).deb (--install):
subprocess new pre-installation script returned error exit status 1
Errors were encountered while processing:
/home/julian/Downloads/gog_star_wars_dark_forces_1.0.0.9(1).deb

Any ideas guys??

Try downloading it again. You can always extract it manually. Also, check the game page for needed dependencies.

How do you install it? I have Xubuntu 14.04 and there are no installation issues whatsoever.

Just double click on the downloaded deb file and it'll launch Ubuntu software centre and then go straight into the installation. Except for the fact that it won't install on my system.

Does anyone know how to install from a .tar file?

Try installing with sudo dpkg -i from the terminal.

No need. Just extract it & run.

No need to install it:
extract the archive with file-roller, go to this folder:
/gog_star_wars_dark_forces_1.0.0.9/opt/GOG Games/STAR WARS Dark Forces
and click on start.sh file