If it is a widget style, you need to compile and install it. Untar it to any directory, open a console there, run "./configure --prefix=`kde-config --prefix`", then "make" and then "su -c 'make install'" (enter root password when prompted). Restart KDE, launch Control Center, under Appearence select Style, select it from the popup list, click "Apply".
If it is a theme, open Control Center, under Appearence select Theme Manager, click "Install new themes", select the file. Viola!
It's not the size of a hammer that counts. It's how you wield it.