You can find your account identifier by first tapping Menu, then Options and finally About. It's the string starting with a # sign in the top left corner of the screen.
In case you are unable to login, you can find it in the bottom right corner of the screen during startup.
Please see the image:
1. Log in to your Empires & Puzzles game on your device.
2. Click Menu (on the right hand side of the screen).
3. Click Options (below on the screen).
4. Click About (on the top right hand side of the screen).
5. Check your Account ID on the top left hand side of the screen: #xxxx