Without knowing what product versions, platform versions, deployment configuration, and other details surrounding what you are doing, we cannot offer anything specific.
The only specific things I can offer based on what you provided is to say that if you are using Forms 9 or newer, there is no native way to change a button label. You would need to use a java bean/pjc. This would need to be custom designed. There are some bean examples on this non-Oracle site:
As for client/server (Forms 6), you can use the WHEN-MOUSE-ENTER trigger
Do you want to just display text in a "Bubble Hint"? If so, open the Property Palette for your button and scroll to the bottom of the list of properties until you find the "Tooltip" property. Now, enter the text you want displayed when a user hovers the mouse pointer over the button.
I want when I put the mouse "*on*" button, item_text be visible (using set_item_property ('block.item' ,visible, property_true) ... I just need to trigger name!! I tried with when-mouse-enter but without success
Please go back and read my first reply. You still have not mentioned which Forms version you are using. If you are using any browser based version, which would be 9.0.x or newer OR 6.0.8 web deployed, then you cannot use WHEN-MOUSE-ENTER. In this case, you will need to use a bean. Refer to the web site I mentioned.
For client/server versions, 6.0.x or older, WHEN-MOUSE-ENTER can be used. Further, statements such as these "...without success..." are not helpful. What does this mean? Was there an error message? Does nothing happen? What exactly did you try? Exactly what Forms version are you using?
Do do you mean - when you click on the button with your mouse or are you talking about just moving the mouse cursor over the button?
The first option is easy - you use the When-Button-Clicked trigger. This will execute the code of your button regardless of how the button is activated (mouse click or keyboard).
If you are looking for "Mouse Over" type of functionality then you can't implement this type of functionality "natively" in Oracle Forms. You can do this using a JavaBean, but you will have to create the Java Bean yourself or find one that has already been created. Google should give you some results on this.