I will make the import dialogs more clear about this.
The database import is for files saved by DC - exported monsters,items,spells.
The UA monster DAT files should be imported using the menu File->Import, select monsters, check the 'All in folder', browse for your folder with all 127, and they should import ok. I noticed that there was one monster that won't import, I'm not sure why yet.
You can use this procedure to import UA items also.
After importing UA files, you can use the database options to export them to new items.txt and monsters.txt files, which can become your defaults if you like. If you don't export them to the default files, they will still remain with your saved design.
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CocoaSpud
Dungeon Craft Development Team
http://uaf.sourceforge.net