Good news! Your Elster A1100 meter is compatible with the Powerpal
The pulse rate to enter into the app is 1000 and you can use the rectangular mount to attach your Powerpal to your meter.
Simply attach the mounting plate over the centre LED (marked 1000 imp/kWh) on your main consumption meter so you can see the light flashing through the hole in the bracket. Then gently twist your Powerpal's sensor clockwise into place.
See the image below for how everything should look once installed.
Then head to https://www.powerpal.net/install and follow the instructions to complete your installation.