Once again, it's frustrating just how rough the API is. The documentation is incomplete and contains errors, and some of the API calls simply don't work (see this post). I know I'm sounding like a broken record, but this API really needs a test suite. The quickest way to annoy potential users of the API is to get them to find really obvious bugs for you.
With a test suite in mind, I've created a simple app that enables you to connect to your Mendeley account and perform a bunch of simple tasks. The hardest part of getting this working was getting my head around OAuth. Luckily, @abraham has written a PHP library to support OAuth access to Twitter's API, so I grabbed that and replaced Twitter-specific code with the equivalent code for Mendeley.
Demo
You can try the app here: http://iphylo.org/~rpage/mendeley/moauth/.
The first time you go to the app is shows a button to connect to Mendeley. If you click on it you'll see something like this:
(if you're not already logged in to Mendeley it may ask you to log in — note that all of this happens on Mendeley's site, my app never knows your username or password details). If you're willing to try the app, allow it to connect to your account. You'll then see a bunch of API requests and results. All but one of the requests is simply displaying information. One request does try to add a test document (the one listed on the Mendeley developer's site), but at the moment this part of the API doesn't seem to work (nor does the call to get the list of papers that you've authored).
If and when Mendeley get the API working fully (and documented) there's a lot of scope here. But what I'd really like to see is Mendeley develop a test suite that runs through every API call and checks that the methods work as advertised.