Well you're using IIS for starters which if not configured correctly can cause headaches for you. The first problem I ran into vising your forum was that FastCGI has a full pool queue.
Quote:FastCGI Error
The FastCGI Handler was unable to process the request.
Error Details:
The FastCGI pool queue is full
Error Number: 4 (0x80070004).
Error Description: The system cannot open the file.
HTTP Error 500 - Server Error.
Internet Information Services (IIS)
Since your using IIS, the server permissions will be different. The system is more than likely Windows, and so the permissions will likely be set up differently than what MyBB generally provides in the readme.
As for the "install" folder not working, ensure that "install" is located in your "forum" directory. Check to make sure your download included the install folder as well, which is usually located at the root of the compressed file. Someone with more IIS experience will have to help you with anything IIS related though or you will need to contact your hosting provider to find out what you need to configure for MyBB to run on your server.
Many thanks Vashnik,
It turns out I was on shared hosting so had no ability to change my file permissions (it looked like I had done in FileZilla then they just reverted back!). I will be sure to install MyBB as soon as this is sorted.
Best wishes