On the left side of your device you'll see the SIM card slot. Insert a SIM Card ejection tool (included with your Travis Touch Plus), in the small hole underneath the slot, press gently and the tray will pop out.
Then take your nano SIM card, put it on the tray and insert the tray back in the device with the metal part facing upwards.
Once you put it back in and the device detects it may ask you for the pin code. Type it and you are good to go.