Some phone makers (Sony & Samsung) and Amazon with the Kindlefire have been overwriting the default keyboard input with there own removing this option on the simplified numbers only keyboard the application uses by default.
Not to worry as since version 2.0.2 there is a special setting to help people.
Simply visit Global Settings in More and enable the “Alternative Keyboard” option. You will then get a new different type of keyboard when using the calculator that will work perfectly for you.