the code for the info window is this:
infowindowtext: '<span style="font: 12px Verdana, Arial, Helvetica, sans-serif; color: black;"><?php echo str_replace("'", "\'", "<strong>".View."<br/>".$row_rsCentres['activityname'] ."</strong><br />".$row_rsCentres['phone'] ."<br/>".$row_rsCentres['add1'] ."<br />".$row_rsCentres['town'] ." ".$row_rsCentres['postcode'] .""); ?></span>',
change it to this to add your link:
infowindowtext: '<span style="font: 12px Verdana, Arial, Helvetica, sans-serif; color: black;"><?php echo str_replace("'", "\'", "<strong>".View."<br/>".$row_rsCentres['activityname'] ."</strong><br />".$row_rsCentres['phone'] ."<br/>".$row_rsCentres['add1'] ."<br />".$row_rsCentres['town'] ." ".$row_rsCentres['postcode'] ."<br /><a href=\"activity_details.php?actcode=".$row_rsCentres['actcode'] ."\">View</a>"); ?></span>',