In order to create tooltips on layout objects such as buttons you would need to have FileMaker Pro Advanced. Regular FileMaker Pro users will be able to see the tooltips once you add them, but you cannot create tooltips in FileMaker Pro.
If you have Pro Advanced, you need to go into Layout Mode (Win: Ctrl+L or Mac: Command+L) and then highlight the layout object you would like to add a tooltip to. Next go to the Format Menu and click "Set Tooltip". You should now see a window where you can specify what you want your tooltip to say.
New features in FileMaker Pro 10
• Layout object tooltips — You can now use FileMaker Pro to add descriptive text that appears when users move the pointer over layout objects in Browse or Find modes. See Adding tooltips on layouts.
To be clear about what Daniele has posted - the ability to create and edit tooltips is now a feature of both FileMaker Pro and FileMaker Pro Advanced in version 10.
Ah yes thank you raybaudi!
It slipped my mind that we brought this feature to Pro when we released FileMaker 10.
To be clear about what Daniele has posted ...
I was vague on purpose ;)