I am trying to make a skin, but I can't find thecategory page with the URL's.can somebody help me?

It's here. Browse through the list or the alpha list on the left to find your series category pages.

thanks,but how do i get the URL?

i dont really understand what url do you refer?
just do as tama's post has told you, and get to the category gallery page
i take an example
you have your skin is an image of jigoku shoujo then you go Browse, see in the left to find Alphabetical Listing, go to the J letter,
then here's the link of jigoku shoujo: http://browse.minitokyo.net/1136/jigoku-shoujo/
see it, the ID is 1136, type it to the feature blank in your skin builder.

same for other category