Try this DHT sensor example. It shows you how to power the switchable Grove connectors, and you can just replace the DHT stuff with some SoftwareSerial code to use either the D6-7 or D10-11 pins for the serial comms.
Or you could use the D4-5 connector, the default jumper sets it to 3.3v constant. The D6-7 and D10-11 sockets are always switchable, either 3.3v or 5. Only experienced users should use the 5v option, since they will need to insure that the sensor or other peripheral device is either one-way or has level converters so that the higher voltage required by the remote device won’t get fed back to one of the Mayfly digital pins.