It helps to push the action button on the siren multiple times (sometimes 6-7 are needed). Here are instructions I received from The Smartest House that solved my hub pairing issues with the siren:
The key with pairing the siren correctly is to holding it close to the hub during the initial set up process, as you have been doing.
When you plug in the siren, please press and release the action button and check whether the led light is blinking or is it solid.
If it is blinking, please keep the siren plugged in and select from the STs menu “add thing” a then quickly press action button at least few times. Sometimes it requires 6-7 clicks before a device can be recognized.
If the led remain solid, then please try general exclusion:
1.Tap the menu
2.Tap the Hub
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.While Exclusion Mode is active, perform the device-specific exclusion confirmation (e.g., a physical button press). You can find instructions for excluding certain compatible Z-Wave devices in the Things category
7.The app will confirm device exclusion