Was that a wide angle converter or something like that? I only heard of Grobartig wide angle converter. If that is the case, just buy one from ebay, it don't cost much.
And if it was an actual camera lens (eg. Canon 17-55mm, Nikon 17-55mm etc etc), all you need is to know the thread size and go to ebay and get one easily.
If it's the front cap you can use any no name lens cap of the same diameter. Better return it soon, these adapters are just a bad joke ... Search for Rainox close focus lenses, you'll have better results.