Donna,
First, welcome to Writing.com (aka WDC)!
Strictly for the purpose of competing in the weekly contests, the Saturday assignments need to be posted in your portfolio. On the left-hand side of the web page, you can find a big green plus sign with the words "Create A New Item". You would then create a 'Static Item', which is a generic term for a single, stand-alone posting on WDC.
You don't have to post anything else; doing the assignments is on the honor system. You don't have to post the Saturday assignment either, if you aren't entering it in the contest.
If you do want to enter today's round, then before 12 noon WDC time tomorrow, while you are using the entry form to log your assignment, you need to include the item number (without the '#' sign) of the static item you created for your Antagonist backstory, check off that it's an item id (an entry id is something more complicated) and that you are entering it in the contest.
As a free member, you only get 10 items in your portfolio. You can delete old ones to add new ones of course, but there's no point in using up your items for regular assignments. Save that for when you want to ask someone to look at what you did and get their opinion (which is essentially what you are doing if you are entering the contest round of course.)
...
Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge
"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh
āNo, writing 50,000 words in a month is
normal. You are
not crazy. This is
not insane.ā ~Teri Brown