Cannot upgrade from Filemaker Server 10 to 11 due to error

Hello all,
I am facing a problem. The Filemaker Server 10 is running on the background of MacServer, but I'm unable to open its interface for customizing it. That’s why I wanted to upgrade it to FMServer 11 (already bought the software), but it doesn't allow me to do it. Instead, I'm getting this error message:
"FileMaker Server 10 is installed. To upgrade to version 11, follow the instructions in the Getting Started Guide."
Can anyone help me of getting rid of it? Thanks in advance.
