Well in the Piezoresistive effect page it does say:-

This section's factual accuracy is disputed.

The only thing that is disputed is whether metals are subject to the piezoresistive effect or not.
Reading the entire discussion, including the linked pages, results in a much more informed opinion.

Quit the IDE.  Unplug EVERYTHING from the board.  Hook the usb cable back up, load the IDE, and try again.

If that fails, try pressing the RESET button just before clicking on UPLOAD.

There is nothing wrong with the bootloader since it works in the Parallels install of Windows.

Create a new user on the Mac, install the Arduino IDE in that account, and try uploading. 
This will tell you if the problem is related to the original user profile, or if it's a system-wide issue.

Also, the Mac can be particular regarding which usb port you are using.  Are you connecting to a port
on the keyboard, or on the CPU itself?

Go here: /Users/<USERNAME>/Library/Arduino/preferences.txt, delete the preferences file, reboot, and try again.

Can you please post the code that you are using?

The middle pin on the Tinkerkit connector is the signal pin, that would hook up to the digital pin that you want to use on the Arduino.  The Tinkerkit relay is a module that has the necessary transistor for switching it, as well as status LED's.  Just hook the + and - pins up to 5V and ground, and the middle pin the digital output pin that you want to use.

My Mega2560 works just fine on my Mac with Lion.

Take a look here : http://arduino.cc/hu/Guide/MacOSX particularly sections 4 and 8, and see if that helps.

Did you extract the Morse folder from the zip file, then copy the Morse folder to the Libraries folder?

My sketch folder is Documents -> Arduino.  Then I have Libraries, so the the full path is Documents -> Arduino -> Libraries -> Morse.

Is that what yours looks like?

Definitely a weird file structure.  The mac Arduino download is a zip file with just the Arduino application inside that should be dragged into the applications folder.

Worked fine on OS X Lion using 1.0 for me as well.

I think that leaves your only option as using a weight-related sensing setup.

That could be a lack of resources issue.  Have you rebooted since this happened?

Seeing the code would be the first step to trying to help figure this out.  I know that you checked it - but I have checked my own code before and missed something.  A fresh set of eyes is always a good thing.

You definitely need a breakout cable or board to program the Freeduino Lite.

Straight from the Freeduino site "...easy programming connection via FTDI Breakout cable or Bhasha's own USB breakout board"

The only time that I have seen OS X only show the CANCEL option when choosing what program to open a file with was when a non-executable file was selected.

Make sure that you are clicking on the arduino application and not a zip or dmg file.