I was finally able to upload a sketch!
1- I could flash the new firmware by pressing the RST button in the board several times, while clicking once on the green button of the FirmwareUpdater. It’s a bit of hit-and-miss but at some point it worked.
2- To upload a sketch select the port “/dev/cu.usbmodem1423” as shown in the pic (I’m not sure why, but the port showed up with the switch of the Linkit to “MS” but then I can still upload sketched when it’s set to “UART” mode)
3- To watch the serial monitor, select the other port “/dev/cu.usbmodem1421 (LinkIt ONE)” and then open Arduino’s serial console.
Will now try to play with the GPRS modem to connect to http://ubidots.com