Use the search facility, which you invoke by tapping the round "Home" button. Hit Home once if you are on your home screen, or twice if you are currently not on your Home screen. Enter "IBKR Mobile" and hit Search.
If you still cannot find the application, reboot your device by holding down the home and power buttons together until the device shuts down. Then hold the power button to re-power the device.
In some cases, the application can get "stuck" in a state where it is only half installed. Try going to the app store and re-installing.
If you still cannot get the IBKR Mobile app, please call Apple support for assistance.