I noticed when you assign your link you used .$siteUrl. have you actually set that variable?
Also since you would be comming from an outside source (e-mail hosting website) it would work just as fine to "hard-code" your link as your full website address. Ex:
<a href="http://www.yourwebsite.com/yourdirectories/activate.php?username=".$signup[username].">Click Here</a>
So a value of your message value could be.
$message = "Hello ".$signup[fname].",<BR>".
"Get ready to start getting the hits you deserve. Now here is your login info:<BR> <BR>".
"username: ".$signup[username]."<BR>".
"password: ".$signup[password]."<BR><BR>".
"<B>The next step is to click on this link to activate your account:<a href='http://www.yourwebsite.comactivate.php?username=".$signup[username]."'>CLICK HERE</a></b>";
You shouldn't have a problem with the double slashes but if you have a problem with those you can use:
$message = "Hello ".$signup[fname].",<BR>".
"Get ready to start getting the hits you deserve. Now here is your login info:<BR> <BR>".
"username: ".$signup[username]."<BR>".
"password: ".$signup[password]."<BR><BR>".
"<B>The next step is to click on this link to activate your account:<a href='http:/"."/www.yourwebsite.com/activate.php?username=".$signup[username]."'>CLICK HERE</a></b>";