Try this :
Locate and "Open" up "Firefox", Look at the upper left hand corner and find "Tools" highlight and click to open, Scroll down to "Add-ons" highlight and click on it, a new window will open up. Locate "Megaupload Toolbar" and click on it, now it should be highlighted, click "Enable" now if you have been using a Proxy add-on tool like "SwitchProxy Tool" disable it(why you ask, because it will not work with the Megaupload Toolbar) and close the "Add-Ons" window.
Now close your "Firefox Browser" so the new settings will take affect. Now open up Firefox again and now you should have the "Megaupload Toolbar" showing. If not then make sure all "Firefox" windows are closed, even the "Download" window, anything with the "Firefox Icon" in your "Task bar", then open up "Firefox" again.