You have a couple different ways of finding your Machine ID, and both are fast and easy.
Option 1: Use the NinjaTrader Help Menu
Open your NinjaTrader Control Center and click Help->About...
This should bring up the About NinjaTrader window. You should see a line item called Machine ID with a 32 digit code. That 32 digit code is your Machine ID.
Option 2: Use the ShowMyMachineId indicator
If you haven't done so already, download and install the free NinjaTrader indicator called ShowMyMachineId. If you're not sure how to download and install the indicator, click here.
Add the indicator to one of your charts. Click here to see how to add an indicator to a chart.
You should receive a popup window that will show your Machine ID. When you click OK it will automatically copy your Machine ID to your clipboard, which means you can go and paste it somewhere else if needed.