I have not tried downloading or using that addon, but general suggestions:

• Customise: look in the menu options is it something needing to be dragged to a location you would prefer. See Customize Firefox controls, buttons and toolbars
• Menu -> Addons: open the about:addons page or click the addons icon from the main menu when you look at the extension details does it give options to customise or use the IDE

Thanks for your reply John..but it is not working. I already installed Selenium IDE,but is is not showing.