How do I fix these errors?

Notice: Undefined variable: disp_trackbacks in /home/philgam/public_html/v2/skins/_feedback.php on line 63

Notice: Undefined variable: disp_pingbacks in /home/philgam/public_html/v2/skins/_feedback.php on line 69

Notice: Undefined variable: disp_trackback_url in /home/philgam/public_html/v2/skins/_feedback.php on line 76

Notice: Undefined variable: disp_trackbacks in /home/philgam/public_html/v2/skins/_feedback.php on line 94

Notice: Undefined variable: disp_pingbacks in /home/philgam/public_html/v2/skins/_feedback.php on line 94

It appears when viewing a certain post.
It only shows up with the skin named "Plain".
Skin link -> http://skins.b2evolution.net/a.php?skin=plain

Thanks!