Wait are you talking about Invision 1.x, the old free version?
If so - no way in heck should you be using that, regardless of how good it may be. It was discontinued over 10 years ago and the patches stopped a long, long time ago. In addition to that it's PHP 3.x codebase means it's very inefficient and stands little to no chance of working on a modern host using PHP 5.5 or the upcoming PHP 5.6
Even the InvisionFree hosting service dont really use 1.x anymore, most of what they use is their own in-house product now as it was way too buggy, and full of massive security holes.
Final 'pro tip': avoid anything that makes use of any of the native PHP mysql_* or mysqli_* commands.