thanks for your help,
here is what happened so far:
I have followed the steps in:
http://wiki.mikrotik.com/wiki/How_to_Co ... _xDSL_Line
something very very strange happened!! when I wanted to test if I an access the internet by a laptop which connected to the network, I had many iexplorer pages opened, I tried to refresh them but I couldnt however only one of them I could! I could even browse in this website normally but when I click on an external link it doesnt work. strange isn't it?!?!??!