Here is how to use steemworld.org's manual vote tool for comments.
Expand the comment you want to vote for.
Then right click and copy the permalink
Paste the link in the link box, it should fill out the author box.
Set the slider where you want it, and click vote.
That is how you would vote for a comment that you commented on.
Paste that link into the link box. Note the "z" at the end of the link, and the name that auto filled in the Author box.
Remove that "z", and then look at the Author box to make sure the name changed to the person you want to vote on.
Set the slider and click the vote button.