Thank you for your post.
I have sent you a private message (top of this page - right side - envelope icon just below the blue horizontal bar) with a temporary link to download the FileMaker Server 10 trial which allows you to uninstall FileMaker Server 10.
I have the same problem. I installed the trial version of FMPRO server 10 and thought I uninstalled it completely but now when I want to try Server 11, I can not install because it claims I have a version 10 installed.
please advise or send me a link to the version 10 install so that I can install/uninstall it.