U:  Scrolls up in the Search Path list.

D:  Scrolls down in the Search Path list.

Delete:  Deletes the Search Path that's in the text field to its left.

Add Search Path:  Brings up the file requester to add a Project Directory to the list.

Note about additional search paths:  These are additional Project Directories. If you load a scene and messiah can't find something in your main Project Directory, it will look at these first, before prompting you that it can't find it. The U and D will scroll up and down to additional entries (you can have up to 25 additional ones).