The link to weigh-in is available from the left "User Menu" and is labelled "My Weigh-in".  If this menu is not visible you may need to login first as this function is only available to registered members.

The weigh-in page requires you to enter your current weight in kilograms (kg) in the place provided.  Once you click submit, the data is stored away as your official weigh-in for the current week.   If you wish to change your weigh-in, simply go back to "My Weigh-in" and you can enter an updated weight up till the close of the week for your team.