I'm new to PBS however I've mailed 6 books in 2 weeks but only 4 show up in my account!! grrrrrrrrrrrrrrr...how to get this fixed?

Have 2 of them been received already?

No, they have not been marked received

Look under your transaction archive.  They've probably been marked received. 

I've looked EVERYWHERE and they've gone AWOL....:(

Even if they were cancelled, they should show up in your transaction archive (which is under the "My Account" tab). Check there.

Did you forget to mark them as mailed in time? The order will disappear from all tabs and show in your Transaction Archive with a big red X on it. If they are there with a green check mark, then everything is OK, they were marked received.  If they are there with the big red X and the details say that the books were not marked as mailed, then PM the sender to let them know that you did in fact mail them, and ask that they mark them as received from their transaction archive when they get them. If they end up with 2 copies because the system reordered the book for them, then they are not obligated to mark yours as received if they were not marked mailed in time.