Create a story with advanced editor mode. Create a variable, say, VARIABLE, and define it as equal to 1 or 0 or whatever as its initial value. Create a new page and add it as a link. Then, click the little Stop sign in the editor on the page that corresponds to the new page link. When you do, a small window will pop up with the options "link options," "variable restrictions," and "item restrictions." Under variable restrictions, you will then scroll to your variable, and then select a value the variable must be for the link to be visible.
For example, if in variable restrictions you require the variable to equal 1, you will select the equal relation and type in the number 1. Then, in other parts in your story, if that variable becomes equal to 1, a reader will be able to see the link. If it does not, the link will be invisible.
I highly advise you to read all of the Help & Info links. I must have read them all by my second week here, and that stuff is the minimum information necessary if you don't want to type the same page ten times with slightly different text.
A couple useful tutorials:
How to Use Chapters, Pages, and Links
How to use Link and Variable Restrictions
