well, assuming that you have already signed as as
a member (you have to be a member first) then you
go to "My page" section and you will find a link
on the top right that says something like
"Register big web auction". Then they will ask you
to enter your credit card if you don't have
already one registered and it's done. They won't
charge the $5 instantly but in about 1-2 months.
Anyway, if you register and pay those $5 make sure
you bid on something or else you would have really
wasted them (they charge the fee even if you don't
bid or win anything)