Under your user name at the top (where you mood is) you will see your new notifications. When you get a new rep comment is says so. Click on that. When the page opens scroll right to the bottom and you will see them
at the top right where it says welcome b_n_3003 under that it should say 'rep coments' or something to that effect. click on the link and it will take you to another page. scroll all the way down and you will see the comments in a box at the bottom.
to give a rep comment at the very bottom of someones post you will see a green/red light to tell you if thery are online then a white scale thing and then a triangle sign. click on the scale thing and a box appears you write in there.