Luckily they have a very good guide on how to attribute[1]. If you are using them on a site or in an App you can simply put the attributions in your about page.
The requirements listed on that page are still kind of silly, though, especially for a commercial project, where it requires you to list the attribution right near the use of it. For example: "If your product is listed online, you must include the proper attribution on the same page next to the item you are selling. "
That kind of requirement is just begging to be ignored completely.
[1] http://blog.thenounproject.com/post/12554806140/the-noun-pro...