IOError: [Errno 2] No such file or directory

[hwiguna]

I'm following along this tutorial on learn.adafruit.com.
http://learn.adafruit.com/matrix-7-segm ... pi-for-i2c

When I run the example:
sudo pytho ex_8x8_pixels.py

I receive this error:
IOError: [Errno 2] No such file or directory

Typing sudo i2cdetect -y 0 also returns a similar error:
Error: Could not open file '/dev/i2c-0' or '/dev/i2c/0': No such file or directory.

The installation of python-smbus and i2c-tools succeeded. What did I do wrong?

[adafruit]

are you using Occidentalis?

[hwiguna]

I was using Raspbian Wheezy and installed packages as outlined in the tutorial.
I've since tried Occidentalis, but my SD card (4GB PNY P-SDHC4G4-GE) doesn't like it. It failed during boot with:
PANIC: VFS: Unable to mount root fs on unknown-block(179,2)

I've checked the md5 on the .img. It matches, so it must be the card. I'll try imaging it from a Windows machine.

[hwiguna]

Finally got around to setting up Occidentalis from a Windows PC. The card kept saying that it's write protected eventhough it is clearly NOT in the lock position. Google found this thread about putting a tape over the write protect tab. It worked! Occidentalis is great! I was able to get the I2C to work out of the box. Thanks AdaFruit!