Haven't I replied to this a few threads ago?
If so... This will now definitely cost you a star!
What other reason have I to answer all these questions? (Tutorial on giving out stars will soon follow!...)
So that the on mouse event be available, you have to be setting actions in the instance properties & NOT in the frame properties! Check the title of that box! You usually get there by right-clicking select your button, followed by selecting properties in Flash 4 or actions in Flash 5.
Another reason why the on mouse event is often not available is that your button's behavior hasn't been set to button. You start off by creating a new symbol and designing your button (or drawing the graphics of your button and then converting it to a symbol), in each case giving hit a name and setting it's behavior. It is that symbol that you drag on the stage. If right-clicking this symbol followed by properties or actions doesn't bring up the instance's properties box... There's something wrong somewhere! Once again from the library (CTRL L) when you right-click your button, you can change it's behavior, if you've made a mistake.
Next question?
;-) "If nothing else, I can always serve as a bad example!”