Will this fit my cameras?

I have two different cameras, and I was going to buy the fish eye add on filter lens. One is a Canon XSi, and the other is a Nikon D60. I don't quick understand how these will attach to my cameras. I understand that they will need to be attached to a lens that is already connected to the body, but how do I know what is the thread size of the adapter I need. I've seen the numbers 58mm and 52mm on these advertisements. Can you explain to me which thread adapter I would need on each? The lens on each of the cameras are 18-55mm. Please and thank you.

These add-ons are actually for my girlfriend's nikon d60 and the other for my dad's canon xsi.

The 18-55 mm Nikkor lens has a 52 mm filter thread
The 18-55 mm Canon lens has a 58 mm filter thread

Lenses from different cameras are not usually interchangeable. Sometimes even different models in the same brand may have slightly different connections. You will need to buy a specific lens for each camera, to ensure it works properly.

Unless it comes with adapters, you'll need to buy one for each filter thread - find this out first (each lens should have a Theta (0 with a line through it) symbol with a diameter in mm this is the filter diameter).

A word of warning, though, these things are junk.