You can't really. There are Conditional formatting tricks combined with If steps in scripts that come close.
If you use layout text for your button, a conditional format can set the size of the text to 500 pts and this will make the text disappear.
If you use a conventional button, you can use a conditional format to change the text color to a dark grey to simulate the 'disabled' look typical in other apps.
In both cases, you put the button's script inside an If block that keeps the script from executing if the button is supposed to be disabled.
This still leaves a spot on your button that will flash black when you click it, but nothing else takes place so this is pretty much the best you can do unless you go to the added extreme of placing the button inside an invisible tab control and using a script with go to object to hide/reveal the button.
FileMaker currently (up through vs. 11) does not have ability to display or hide controls. You can 'hide' text, fields and buttons with conditional formatting but the object remains on the layout. The way it is usually handled so a button is not even clickable or a pop-up never pops until/unless needed, is to put the controls in a one-row portal and use calculation test on whether to display them or not.
Being able to set control properties is such a basic function of programming. I'm surprise FileMaker has many controls for designing an interface with no simple way to enable/disable them. Is there a place to request wanted features in future releases?