when can i start rating comments?

i also experienced that but at that time, i still don't know what Discussion Rating System is or the mylot reputation (star). I also can't find any appropriate answer to your questions but after i reached 100 user activity ratings, i can already use it and the rating that i gave will stay and will not come back to the + - !

You will be able to rate when the number beside your name reaches 100. By that time you will have understood (hopefully) how to use the rating system and all of the other things that there are here on myLot. The rating buttons are not to be used in the same way the the 'like' buttons are used on FB as you can see if you read here: http://www.mylot.com/o/faq/faq17.aspx