This may be template specific, meaning that different templates place the define pages in different sideboxes.
Most often, that particular link is in the More Information side box rather than the information sidebox. Often, the Information sidebox is on the right and the More Information sidebox is on the left.
This too, varies by template. Some templates may not have both.
Have you checked to see which sidebox modules you have active?
In Admin, go to Tools/Layout Boxes Controller and look for a sidebox called More Information
You should not need to do any php file editing to make this show up on the page.
You can make hundreds of changes to your sites appearance through the Admin without ever editing a file. Always look to the Admin panel first, before assuming you have to edit a file.
I've edited many files that I later found out I could have changed through Admin. I learned the hard way that it pays to read the forum in some detail before jumping in to files :frusty:
Consider starting with these:
https://www.zen-cart.com/tutorials/index.php
http://www.zen-cart.com/forum/forumdisplay.php?f=14
http://www.zen-cart.com/forum/forumdisplay.php?f=19
http://www.zen-cart.com/forum/forumdisplay.php?f=136
http://www.zen-cart.com/forum/forumdisplay.php?f=15
http://www.zen-cart.com/forum/forumdisplay.php?f=16
http://www.zen-cart.com/forum/forumdisplay.php?f=27
And very importantly:
http://www.zen-cart.com/wiki/index.php/Important_Site_Security_Recommendations
