Can someone explain to me how the Downline Builder works? Thanks

If you are refering to the downline builder of this
forum there is a pinned topic with the rules at the top
of the folder.

briefly:

if you are interested in joining any program in the downline builder
post a reply saying 'count me in' -or cmi for shorter
once the people who posted cmi inmediatelly before you is done (s)he will post her referrer link
use that link to join the program
once you have joined get your referrer link, edit your post and paste it there -so the next one will be your referral =]

if you change your mind about joining, change the cmi post to count me out
and people afer you will inmediatelly skip you =]

for more information, remember: always read the rules!