Well if you take a "vote" on it 7 Mar is going to win. It's the first date given in the TL, it's the date given on top of the assignment in the TL, and it's the date you'll find on myUnisa.
The only place the 14th appears is the box that says ... :
IMPORTANT INFORMATION ABOUT EXAM ADMISSION
Due to regulations by the Department of National Education, students must meet the following
deadline for this module:
SEMESTER 1: Submit Assignment 01 BEFORE or on 14 March 2011.
If you read it carefully you'll see that this is of the nature of an explanatory note, rather than a directive. "Because the Dept of Education has done this to us ... " simply explains why the assignment is utterly critical. Note that the word BEFORE is in capitals.
I would say the safest is to go with 7 Mar. If that's going to be tough for you, contact the lecturers. Maybe they'll make some kind of exception for you, just because they'd like you to have a shot at passing this.
Well that's my opinion, anyway.
Looks like some lecturers are popping into myUnisa, so if you're shy a last resort would be to post the question there.
I think you'll find that the lecturers really do want you to succeed. If nothing else it's a measure of their success at setting up the course in such a way as to get through to you, and bring you enlightenment.