here is my setup:
i have a linksys router connected to the internet through a static ip.
Local IP Address on Linksys router is 192.168.1.1 subnet 255.255.255.0
i connected my lan port on linksys router to ether1
i saw this video on tiktube.com on setting up a pppoe server here : http://tiktube.com/?video=137
and did exactly the same 
everything went smooth : i can ping google.com from winbox ,so my mikrotik has internet working. but my other computers also connect successfully through pppoe to mikrotik but they dont get a GATEWAY and i am unable to browse on the computers.
Here is exactly what i did after a new install of mikrotik 3.15:
ip address add address=192.168.1.20/24 broadcast=192.168.1.255 interface=ether1
ip address add address=172.16.100.100/24 broadcast=172.16.100.255 interface=ether2
ip dns set primary-dns=192.168.1.1 allow-remote-requests=yes
and followed the whole video till the end.. i completed without any errors
Please tell me what should i do to start my browsing on the pppoe client computers!
regards