These warnings are shown because the resources were missing in the plugin installation process. This is now fixed and uploaded. If you prefer not to download and reinstall the plugin you can simply add the resources:
sevenspikes.jcarousel.admin.settings.common.name.required and sevenspikes.jcarousel.admin.settings.common.skin.required
"Name of the JCarousel is REQUIRED." and "Skin of the JCarousel is REQUIRED."
The resources can be added via the Administration -> Configuration -> Languages and click the "View Resources" link for your active languages.