The information about your open position, including Realized and Unrealized PNL is available on the bottom of the BTCMEX Trading page.
You can also make the calculations of the PNL using the Calculator on BTCMEX. You can find it in the "Place Order" window on the left from the Trading Chart.
Select the "Profit/Loss" to start your calculations.
Learn about the PNL Calculations in the BTCMEX official Trading Guide.