On the main page, click browse and find the file. Then click UPLOAD. If you have multiple files, click the Multiuper at the left.
I find it easier to upload to MediaFire personally. And your files last WAY longer. I used to upload a bunch of Wii ISOs for a couple of sites on MegaUpload, but then, since no one ever downloaded Wii games and only NDS, they just rotted away... =\