previous_post_type_link() not found?

I’m trying to use previous_post_type_link() & next_post_type_link() functions in a custom type template file, but I get this error whenever I’m trying to access these function:

Fatal error: Call to undefined function previous_post_type_link() in

Any clue?
I’m using plugin v1.5.8

Thank you!

http://wordpress.org/extend/plugins/post-types-order/