Well, according to you're current code, any page with session_start() can access the cart array which is stored in $_SESSION['CART']. No special passing is required as long as you have the session_start() on any page that you wish to use them in. With this, you can access any variable set within the session.
Arrays are easy. Just consider them a small collection of mixed data in php, with index/value pairs. Iteration through loops is also easy, depending on what you have done. According to this code, you're session cart is setup like so:
CART => array(
$id => $num,
$id => $num,
So, you have whats called a multidimensional $_SESSION array indexed by CART. Since you're id index can potentially be imxed, this somewhat limits you on you're iteration. You will require either a while() loop to fetch the current value and proceed to the next, something like:
while ($cur = current($_SESSION['CART']))
But that has a pain because you need to separate you're key / value pairs. A better solution is a foreach loop which was created just for this purpose:
foreach ($_SESSION['CART'] AS $itemKey => $itemQuant)
Think of a session as a server side cookie. The user gives you an associated key for the session and any data within that session can be retrieved regardless of from what page, and still access the same values. This is how the builtin functionality of sessions work in PHP.
If you are having great difficulty with this assignment, you are probably better of requesting help from you're instructor instead of consulting with us. You're instructor knows what you know already and knows what the assignment should be like in the end. This should let him better explain how things are done and what he expects compared to us.