Header error… ?

I tried to add this plugin: http://linux.linux.dk/projects/wordcount/ and I’d like to add more but everytime, there seems to be a header error coming up for some reason. I wrote in my header Total words: <?php echo WordCount(1, 0); ?> and added the plugin to the Plugins folder. It shows on my main index (sidebar) but when I want to go to for example the Visitor page, it shows an error: “Total words: Fatal error: Call to undefined function: wordcount() in /home/mslave/public_html/header.php on line 45” … I tried installing a similar plugin last night too but the same thing happens… It shows on the main index but when I click on a link like the Domain section, it shows an error… I’m lost… Please help me!