You'll find that option when you configure the PPPoE WAN connection.
On my DSL-2500U it is to found under:
Advanced Setup -> WAN
Click Edit on the PPPoE connection
Click Next, Next
Now that option should be shown. See a screenshot of how my page looks: http://www.imagehost.co.za/image-FEED_501F9F23.jpg
Do you perhaps have a Telkom branded DSL-2750U?