5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
3V3. A 3.3 volt supply generated by the on-board regulator. Maximum current draw is 50 mA.
You should power that sensor board (Sparkfun accelerometer breakout board) with the 3.3V output of the Arduino Uno.
But how did you connect the 5V SPI signals from the Arduino Uno to the 3.3V SPI signals of the sensor board ?
The sensor might be damaged by now.