What's stopping my starter from getting power?

Part of the starter circuit involves having the automatic transmission being in neutral or park. There's a switch on the transmission that has to be closed by the linkage. Try moving the shifter while you are trying to crank the starter. If you've got a manual transmission, the clutch may have to be pushed in, and again there's a little switch down there somewhere.

If you have a voltmeter, you can see if you are getting 12 volts to the big starter terminal, probably not, and if you are getting 12 volts to the big relay on the starter that closes the circuit to the motor. Perhaps you forgot to hook up the wire to that relay? Or if there is just a big connector plug, take it apart and clean it, blow it out with compressed air and try again.