Navigate to the Main Page and drag the "Enable Flash" link to your bookmarks bar. If you accidently click it while dragging by mistake click the refresh button on your browser. Once this step is done you will not have to do it again.
Now to use your bookmarklet, go to a website requiring flash (you can test it out using the Test Page) and click the "Enable Flash" bookmarklet, then click the "Enable Flash" link that pops up on the screen. The page will suddenly refresh and flash will be enabled. On the test page you should see a little "Flash is now enabled" animation. If you already see it then flash is automatically turned on by your browser.
For troubleshooting see Here