What paypal system are you using? If paypal payments standard:
look for the following code on the confirm page:
(this code is generated using a cart object named "eCart2", your will be slightly different depending on the cart object name)
<input type="hidden" name="on0_<?php echo $eCart2_Index+1; ?>" id="on0_<?php echo $eCart2_Index+1; ?>" value="" />
<input type="hidden" name="os0_<?php echo $eCart2_Index+1; ?>" id="os0_<?php echo $eCart2_Index+1; ?>" value="" />
this is a variable pair used to pass optional information. set the value of the first one to "SKU"
for the second one, in the property inspector, click the lightning bolt next to initial value and select the SKU cart column.