How to advertise forum for free?

Unfortunately no one wants to be the only member of a community, you are at that seeding stage where you need to develop a certain critical mass in order to attract more people to participate. Some operators, particularly processional site flippers will use either paid participants or a data scraper script to 'borrow' content from other forums to give place a more lived in appearance. You probably just have to put in a lot of labor off sites getting relatively low grade backlinks from participating on other forums and commenting on blogs.

you did advertise it just. but you could make some promo vids for youtube then add friends from other computer related channels to watch them and maybe they will advertise to their subs by liking the video and it being displayed on his friends and subs homepage.