• Open the feeds configuration dialog box.
• If your feed is not listed in the active feeds list click Full List button and add it.
• Double click on its line and type the ID / Password you have received from your data vendor
• Click OK
• Click on your preferred feed and push button 'Set <feed> as Default'
• close dialog box

The default feed is the feed that will be used when the feed parameter is omitted as in the formula:

=P("IBM", "LAST") requests IBM's LAST from the default feed.

While a formula that has a feed abbreviation as a 3rd parameter:

=P("IBM", "LAST", "YDL") requests IBM's LAST specifically from that feed. (Yahoo in this example)