Make sure that when you open the .CSV file it shows the list of computers going down in a row with no commas or other markings anywhere in the list.
Many times the problem comes if the file originated as a .XLS file. When saving a file that has additional settings you need to make sure to answer no when prompted to keep the formatting. This will allow the file to save correctly.
An alternate method to fixing the formatting is to copy and paste the list of computers into Notepad and then save as a .CSV file type. This will work when Excel does not.
One more item of note is that when importing a .CSV file for a collection, it will only add computers that are currently in the database; you cannot populate a collection with computers that have not at least been discovered.
Notification Server 6.0