You need to call the SetValue function of the DevEdit class, which you can do like this:
<?php
include("de/class.devedit.php");
$myDE = new DevEdit;
SetDevEditPath("/de");
$myDE->SetName("myDevEditControl");
$myDE->SetValue("<b>this is the initial value</b>");
$myDE->ShowControl(500, 400, "myimagedir");
?>
You can also set the value of the editor using a variable, such as:
<?php
// Other code here...
$myHTML = "<b>test</b>";
$myDE->SetValue($myHTML);
?>
