Click on the digits (or [.]) to place a number into the x register. Click [ E ] to enter the number. Subsequent clicks on [ E ] will push the x value to y and lift the stack.
You may also click in the register field on any register and enter a register value with the keyboard. Editing can be done just as with any text field.
[Kbd] will place the cursor at the x register for keyboard number entry.
The "Enter" key will lift the stack and place the cursor at the x register. You can then enter a number from the keyboard. To enter another number, press "Enter" twice after the first number.