have a look at this thread:
iTunes metadata
If you want to import data you have to supply data that has a single line per record. One record describes the contents of of all fields for one file.
So youhave to replace the line breaks with something that does not break the line (e.g. the underscore) prior to import.
The import works for multiple files and applies the the data from the first record to the first file in a selection, the second to the second file and so on.
You only have to take care that the data in the text file matches the order of files in the selection.