There are two separate issues with the search facility: (1) No ranking of results by relevance, and (2) no support for special characters (ü, é, etc.). For these issues to be fixed, Giant Bomb (the site that powers Darkadia) will need to change the way their API search function behaves. The lack of special character support would be less of an issue if the search function was able to search across Giant Bomb's game aliases, but that's not supported either.
You might not be able to retrieve those games using the search facility, but they DO exist on Darkadia. If you can find the game's detail page on Darkadia, you'll be able to add it to your library directly from that page.
Here's how to find the game's detail page on Darkadia:
Find the game on Giant Bomb, and take a look at the URL. For example, this is the URL of Brütal Legend:
The bolded bit in that URL (20700) is the game's unique ID, and can be used to navigate directly to the game's page on Darkadia. Continuing with our example, Brütal Legend's page on Darkadia would be:
Here are a few popular games affected by the special character issue:
There are some known problems affecting the loading of images in your library when using Adblock on certain versions of Google Chrome. This is down to a bug in Webkit, the engine that powers Chrome. Disabling Adblock for Darkadia should solve the problem, but your mileage may vary. Special thanks to Hexogen for some brilliant detective work in getting to the bottom of this.