How do I get pass the lock after booting with alt?

You have Firmware password protection in Mac OS X ...

http://support.apple.com/kb/HT1352

It would block usage of all the startup keys, like C, N, T, D, CMD+s, CMD+Option+p+r, CMD +v, Option, and Shift, as well as booting from anything but the Hard Drive.

Force Removing Password Protection

1) Add or remove DIMMs to change the total amount of RAM in the computer.

2) Then, the PRAM must be reset 3 times. (Command + Option + P + R).

http://www.securemac.com/openfirmwarepasswordprotection.php

Well, where's the question or step you need more help on?

If you reinstall the old drive disable Firmware protection booting from that Disk before putting the new Drive in. 🙂

First you will need a Drive in the computer. Second, BDAqua is talking about Memory (RAM) when he refers to "DIMMs" Then follow the instructions. My suggestion would be to remove one of the DIMM sticks thats easier then buying another stick. Just my two cents.