It would be nice if you had not relied on previous experience for documenting how to add media. For example, you can add all the media for the distribution (excluding PLF) with one command, e.g.:

$ urpmi.addmedia --distrib http://my.mirror.com/pub/linux/mandriva/official/2007.1/i586

The GUI way with Mandriva Control Center is the same. From the main Media manager screen, click "Add", then choose "Distribution sources", and choose a mirror.

Note that all available media are added, but some are disabled by default (specifically the backport and test media). They can easily be enabled from the Media Manager.  Adding the --interactive flag to the urpmi.addmedia commain will allow you to choose which media should be enabled when initially adding them.