I am unable to add a definition when Skritter doesn't recognize a word. It works when Skritter is simply missing the English definition. But it doesn't work when the dialog box with the pinyin, definition and traditional variant fields opens up. The error message says "* field required; language definition." However, I've completed the definition field. I've tried Google Chrome and Explorer with no success.

I also have the same problem: "field required; language definition" - doesn't make sense.
A bug in the program source code somewhere?

Yep, a bug I introduced yesterday along with a small tweak to the way words are added. I've made the fix and uploaded it so should be fine now. Thanks for the report you guys!