So you don't see it in the URL?
Well then if that isn't in the URL.... then you missed the step in dosearch.php where you modify $prevlinkstart and $nextlinkstart.
I just checked the instructions... it's there. Check those instructions again.
Especially this part:
## Change this:
$prevlinkstart = "<a href="{$config['phpurl']}/dosearch.php?Cat=$Cat&Forum=$URLForum&Words=$URLWords&Match=$URLMatch&Searchpage=$prevpage&Limit=$Limit&Old=$Old">";
## to this:
$prevlinkstart = "<a href="{$config['phpurl']}/dosearch.php?Cat=$Cat&[:"red"]displaytype=$displaytype&[/]Forum=$URLForum&Words=$URLWords&Match=$URLMatch&Searchpage=$prevpage&Limit=$Limit&Old=$Old">";
## Change this:
$nextlinkstart = "<a href="{$config['phpurl']}/dosearch.php?Cat=$Cat&Forum=$URLForum&Words=$URLWords&Match=$URLMatch&Searchpage=$nextpage&Limit=$Limit&Old=$Old">";
## to this:
$nextlinkstart = "<a href="{$config['phpurl']}/dosearch.php?Cat=$Cat&[:"red"]displaytype=$displaytype&[/]Forum=$URLForum&Words=$URLWords&Match=$URLMatch&Searchpage=$nextpage&Limit=$Limit&Old=$Old">";
It should work fine to you once you get the variables to pass in the URL.