If you want a mature adult forum then down voting posters who post negative items will ruin a good forum.
If you want a forum of like minded yes men where everything is perfect and nothing is wrong then down voting anything negative will help achieve your aim.
I personally have never given a negative vote to anyone... I only give positive votes and thanks.
To give a positive vote you just open the correct thread and find the post you want to vote for.
In the top right corner of the post you will see a small + and a horizontal bar and a small -.
To vote up click on the +, to vote down the -. If there is a green box or red box then someone has already voted... if there is a green or red box and no + or - then you have already voted for that post.
In a thread you have created yourself all the posts have a thanks option to thank other members for their contribution to your thread.
That is all there is to it.
Please use it wisely and don't punish people for delivering news... good or bad.
If you get a negative score you will not be able to vote for or against others.