If the member does not have any feed back score you will have to find a post that the member has made then click on the zero
So go to the member list - type in the name - when the name comes up click on the name- that will take you to their profile - then click on " Find latest post" - then click on one of the persons response - it will bring you up to that person - which will have their name and a feedback score of 0- click on the 0 - then you will see the option of leaving feedback for that person on the right of the screen
It sounds more difficult than it really is - just takes a lot of words to get it across
let me know if you need anymore help