If you have trouble installing the siren, first ensure that it is has not already been paired with the Somfy One / One+. To do this, go to the "System" menu, and verify that it is not listed.
Next, verify that the siren is within radio range of the Somfy One / One+. To do this, try to install the siren just a few yards from the Somfy One / One+. Go to the "System" menu, tap the "+" sign in the upper right corner, and follow the prompts.
If the battery tab has already been removed, remove the batteries and then put them back in when you are prompted to remove the tab.
Finally, make sure there is not a problem with the batteries by restarting the operation using brand new batteries.
Once the siren has been installed, you can test that it is within radio range by placing it where you want to put it and using the "Test Mode" option from the "System" menu.