Assigning Variables

I think you may be misunderstanding how Captivate works.  You do not assign a variable to a button.  You can only assign values to a variable.  You can also display the value of a variable by inserting the variable in the text area of the button, but that will not automatically hide or show the button (as you are wanting to do).

You can hide any Captivate object by selecting it and then in the Properties tab clicking the small eye icon (near the top of the Properties tab) that indicates whether the object is Visible in Output.  If that icon has a diagonal line through it, that means the object will be hidden in the output.

You can then choose to SHOW the object via Conditional Advanced Actions that are executed by events such as the On Slide Enter event.  You would need to set up a Conditional Action that looked at the value of a User Variable to see if it had a specific value, and if so, execute an action to SHOW or HIDE the object.

If none of this makes sense to you because you are still very new to Captivate and online learning, then you are going to need to do a lot more reading and watching of videos to gain a better understanding.  It can be complex.

Take a look at these videos if you haven't seen them already: 

http://www.infosemantics.com.au/adobe-captivate/video-courses/how-to-use-variables-and-advanced-actions