LyX ⇒ Cannot add Converter Definitions

[kissge]

I was trying to fix the problem that included PDF graphics are rendered poorly (rasterised by some reason).
During the struggle I removed a converter definition in Preferences by mistake.
Now I want to restore it, but the Add button is disabled.
How can I add a new converter definition?
Any suggestion will help, thanks.

OS X 10.10.1 Yosemite
LyX 2.1.2.2