While you can talk by using the two on screen buttons, you may want to map a key on your keyboard to use as a hardware PTT button instead of your mouse. To map the button, go to Tools > Options > Control. Select “Set” next to “Push-to-talk” and select the button to use. You can use two keys, such as Ctrl + Scroll Lock. 

If you're using an external microphone with a button, make sure it's plugged in, click on Set and press the microphone button. Zello should identify it if it's a key stroke too.