If you want to take advantage of the enabled state controlling whether or not the button performs, you need to have your action in the Action event, not the MouseDown event.
There’s a number of great resources for beginners.
First, if you’re an absolute beginner to programming, there’s the textbook. It teaches you to program as well as use Xojo at the same time. Great two-in-one system.
There’s also a manual that comes with Xojo that teaches you the ins and outs of using the IDE, the language, and the features within. You can find that in the Documentation folder next to your installation of Xojo. There’s also an online language reference http://documentation.xojo.com
Lastly, when you’re ready to learn from examples, there’s a folder full of example projects next to your Xojo install called “Example Projects” It helps when you don’t intend to reinvent the wheel. Do not copy and paste code from example projects, that won’t help you at all.