I'm another disappointed new, potential Mac user. In my case the installation failures have apparently been network problems (usually time-outs) while in the downloading virus definitions phase of the installer. It seems to start OK and then peter out. My wifi connection is on the slow side but it's not THAT bad. I went looking for log files in the referenced /tmp directory. It's a Yosemite system and the UNIX-type file system seems to live inside the root-level /sw, but there is no /sw/tmp., even though there are var and bin, etc. [to the other guy, it's not the "temp" folder. It is (or should be) a root-(lowest)-level directory in the belly of the OS named tmp (no "e"), You get into that area (on my system anyway) by opening a finder window, going to the "Go" menu, selecting the "Computer" item and then clicking on Macintosh HD.]
Is there a way to download the current definitions directly elsewhere, say onto a different device and carry them on a USB stick?