First of all, verify that your watch is powered off and open the SIM slot with the screwdriver the watch came with.
Push the SIM card into your watch until it clicks firmly into place. You might need to use a tool, such as the screwdriver. The metallic contacts of the SIM-card should be facing up and the angled corner should be facing in.
Once the SIM-card is properly inserted, you will see a full signal bar on the screen bar.