The message keys are for internationalization. The error message says, that - for some unknown reason - in your installation the internationalized text is missing. Is this a fresh install?
Anyhow, adding texts for the message keys is easy. One approach is to install the developer support package, browse to .../ds, turn-on the developer toolbar, browse to the page where the key is missing. On that page, you can click on the missing entry and provide the label.
