text entry box - how to set up a variable

HI

I'd like to create a text entry box where the user must enter, in this example, the number "85.1" into an input field.  How do i go about setting this up?

I created a text entry field and placed it over the input field. I then created a variable called IncomeTaxActSection, but i don't see where i add the correct value - 85.1 -into the field. 

The behavior then is that the user enters this number and then clicks on the magnifying glass icon to the right of the field to proceed to next slide; or if they enter an incorrect number, they are prompted with a 'hint' caption asking them to enter the number 85.1 into the input field.

I know this must be possible; i just can't seem to figure out how to do this.  Thanks, Marion