get_pagination not working on a custom post type query (using WP_Query)

Pagination is only built “well” in non-custom post-types. Below, I’ll post an article with more detail about the more complex setup required for custom post-type pagination via the WP_Query() method.

But to give the direct answer to your question, it’s not working because get_pagination() is not a method that belongs to the WP_Query() object, rather it belongs to the native WordPress loop and is inaccessible as you have it coded in your question. To set it up properly, see this article:

http://wp.tutsplus.com/tutorials/custom-post-type-pagination-chaining-method/