Another noob seeking firmware assistance: Sprinter/Sanguino

I have recently built a Longboat Prusa from the reprap kit store, and it is working. It is running Sprinter on a Sanguino board. I have it connecting and printing on both Mac OSX and WIndows XP.
The problem I have is that the X-axis output is inverted. I can bypass this by mirroring the STL before export, which prints correctly, but I'd prefer to fix it in firmware so I don't have to constantly doublecheck the axes and cancel print jobs due to directionlessness.
I have followed the directions on how to invert the X-axis but I cannot get the new code to compile in Arduino 1.0.2, which I am pretty sure is the latest version, at least for Mac. It gives me an error about pins something-or-other in the heat bed section of the file, which I did not touch (all I altered was the invert-x-axis = true part). I am not a code guy so I am just following instructions, I assume the instructions are wrong or the software is not happy?
I am considering using Daid's tool at [daid.mine.nu] to generate the firmware but I don't want to kill my machine. If I can't compile default firmware, it might leave me dead in the water.
Any notes on new things to try would be most welcome.

Fom the Sprinter git hub page, note the repeated reference to version 0023.
Many firmwares will not compile with Arduino tools later than this.

Software installation
----------------------

1. Install the required packages (gcc-avr, avr-libc, etc.)
sudo apt-get install arduino-core

2. Get the arduino software version 0023, uncompress it in a directory.
Arduino software v1 has not been tested much, but is known to work with some boards.
[www.arduino.cc]

3. Get the sanguino software, version 0023
[sanguino.cc]

follow the sanguino's readme so that your arduino hardware folder looks like
arduino-0023/hardware/arduino
arduino-0023/hardware/sanguino
arduino-0023/hardware/tools

4. Clone the Sprinter git repository.
git clone [github.com]
Optionally, switch to the desired branch
git branch -a
git checkout THE_BRANCH_YOU_WANT

Arduino 0023 will not open the Sprinter files.

Also, to top it off, makefile is not working either. Syntax error after syntax error. Not sure what else I can do to fix this. None of the Arduino software versions can cope. I also cannot compile Marlin as a replacement option. Same issues.

Marlin certainly compiles under OSX with arduino 0023, it's what I run.

What part am I doing wrong here? I have Arduino 023 installed, have the proper hardware folders set up because I can see my Sanguino in the list, got it connecting no problem, but I cannot find anything usable in the Marlin directory to bring into the compiler. Everything is greyed out.

You load Marlin.pde

Error compiling:

In file included from Marlin.h:23,
from cardreader.cpp:1:
pins.h:765:3: error: #error Oops! Make sure you have 'Arduino Mega' selected from the 'Tools -> Boards' menu.

You have the wrong MOTHERBOARD selected in configuration.h
Unless you have a copy of Marlin specifically for your printer, you're going to have to go through configuration.h and change the settings to match your printer and electronics.
The defaults are not going to be even close.

For a reversed axis, it is probably easier to change in hardware than software!

I've got the firmware compiling in arduino v22 and v23, but both versions crash when I go to upload it.

Why don't you just reverse the x axis connector?
That will reverse it's direction.

" Why don't you just reverse the x axis connector? "

I want to reserve that as a final resort. I'd also like to get familiar with how the firmware works and how it can be modified and upgraded.

Speaking of the wires, which ones would I switch?