The handler will also make sure your Motion Sensor is recognized by SmartThings correctly. But before adding it, check that you're using the SmartThings Classic app and not the newest SmartThings app. Not only is the new app not complete, it doesn't support the addition custom device handlers yet.
Here's how to add the handler.
1. Exclude the sensor from your network and log in here with the same information you use for your ST app.
2. Click on My Device Handlers in the top menu
3. In the top right corner, click on "Create new device handler" and choose "from code" from the top tab menu. You'll be presented with a blank area where you can paste the code for the handler. Please click on this link to access the code, then copy ALL of the code from the page.
Once copied, paste it into the blank window on the SmartThings page.
4. Next, scroll to the bottom and click create.
5. Now click save, then click publish > for me.
Once you see the handler on your list of device handlers, include the sensor using one of the three methods listed here.
And once the handler is installed and the sensor is included to your SmartThings network, click on the ZSE18 sensor in your app and click on the gear icon in the top right corner to access the device's advanced settings. There you'll be able to customize your sensor as needed.
Let us know if you have any questions about the process! We're always happy to help.