Double check that you're using the SmartThings Classic app rather than the newest SmartThings app. Even though the new app is available in the app store, it is not currently complete and does not support all devices. Until the new app is ready for all users to enjoy, Classic is the way to go. You may find that you can include your ZSE18 Motion Sensor easily on Classic after failing to do so on the new app.
If you're already using Classic, exclusion is the next thing to try. Exclusion is a soft reset that helps with a variety of configuration issues. Even if your sensor was never included to SmartThings, you can still exclude the device. Here's how to do it:
1. Tap the menu icon in your SmartThings app
2. Tap Hub is Online
3. Tap Z-Wave Utilities
4. Tap General Device Exclusion
5. The app will show this message: “Please follow the manufacturer’s instructions to remove the Z-Wave device from My SmartThings.” There will also be a spinning circle next to the message “Exclusion Mode Active”
6. On your sensor, press and release the Z-Wave button THREE times quickly
7. The app will confirm device exclusion
8. Tap Done
Once exclusion is completed, try including the sensor again in your SmartThings Classic app.
If excluding and re-including the sensor didn't help, please let us know! Our team is ready to help solve this.