We have an API and a set of WordPress-style functions you can use to work with the WishList Member code.
Our API lets you connect to and interact with WishList Member using an external (outside of WordPress) script that you write.
The available WordPress-style functions let you connect to and interact with WishList Member inside of plugins and themes that you write (that will be installed in the same WordPress install as WishList Member).
The documentation for both are available at: http://codex.wishlistproducts.com
The API key is available in the Settings > Configuration > Miscellaneous section of WishList Member.