You can find all the information you require in the PSoC MiniProg3 Program and DebugKit Guide.
Refer to section 3.2.1 for the 5 pin connector pinout.
You need to connect all five pins:
1 - VTARG = Target voltage, in your case it seems like this will be 3.3V.
2 - GND This will be the ground connection you were asking about.
3 - XRES External reset CY8C4125LQI-483 pin 30
4 - SCK Serial Clock CY8C4125LQI-483 pin 13
5 - SDAT Serial data. Can be referred to in a number of way, SDATA and SDIO are common. CY8C4125LQI-483 pin 12
I hope this helps.