This is one of those Dreamweaver errors that has been around forever without a solid fix.
The best work around that I have found is to temporarily detach the page from the template. To do this, close the page in Dreamweaver, and open it in a text editor, I prefer Textpad.
Just after the <head> tag, is the tag that attaches the page to the template, it will look like:
<html xmlns="http://www.w3.org/1999/xhtml"><!-- InstanceBegin template="/Templates/Main.dwt.php" codeOutsideHTMLIsLocked="false" -->
Cut the template tag;
<!-- InstanceBegin template="/Templates/Main.dwt.php" codeOutsideHTMLIsLocked="false" -->
and paste it into a new document so you can add it back in later.
Save the page, close it in the text editor and open it in Dreamweaver to add the Validation Show if. When you are done, close it in Dreamweaver, open it in your text editor and paste the template tag back in after the <head> tag to reattach it to the template.