SOLUTION:
An UNO (or any other board) does not output a (5V) signal. I2c works in tri-state connections. To send a 0-bit, it will pull the SCL or SDA line to GND, to send a 1-bit it will leave the line floating. Hence you always need pullup resistors to bring the line high. I have checked the schematics and these pull-up resistors (1k) are already on the MPL3115A2 board and connect the VCC ( 3v3).
Try to connect the board GND- GND, VCC -3V3, SCL to SCL, SDA to SDA ( NO resistors needed).
Thanks to paulvha from the Sparkfun Forum for helping!