Hack-o-Matic is just an elaborate hex editor.
The basic idea is to open a ROM binary and look for patterns that resemble the graphics in-game. You need to make sure you're in viewing it in "bit mode", so you can see the bit patterns of the graphics. Hex-editing isn't too useful for anything else other than graphics though, unless you know the location of bytes that control other things. Usually a disassembly of the ROM aids in that, but you need a decent understanding of assembly and the 2600's internal workings.
If you really want to do something substantial, find a game that already has a commented disassembly available, grab a copy of the DASM compiler, and start modifying code. That's what I did.
Edited by EarthQuake, Sun Nov 30, 2008 7:41 PM.