Since it is working for me, I assume there is something wrong with your browser settings. Make sure you haven't referrer tracking disabled (can be done with Opera at least, not sure about other browsers) and that you don't click "Download" and immediately change to another browser window the script might catch as referrer then.

This message has come up, when i try to download the actual comic:

Warning: Offset not contained in string in /home/creosote/public_html/comics/download.php on line 83
Error: You've come here from a site which links directly to our files. This is considered linkstealing and we do not allow this. Please report where you came from in the attempt to download! To download the file, go here.