Open the iSeek menu, and select Preferences. Click on the search sites button. Create a New Category or Site by clicking on the New: Category or Site button. Remove an old Category or Site by selecting it and clicking on the Delete button.
You can go to the Ambrosia database of iSeek URLs http://www.ambrosiasw.com/utilities/iseek/weblinks.html. Just click on one, and it will be added to your iSeek menu.