[Lew]

1: Create your keyword and select "Done entering keyword"
2: On the "Create Wishlist Search" screen press "OK" on the keyword field
3: Scroll down to the keyword you just created and press either thumbs button to exclude the keyword (adds a minus sign), again to make it optional (adds parentheses), and a third time to revert make it required (removes symbols).

This should work on all of the fields except the "Wishlist Name" field.