The PSE9 serial number will not work in PSE11. Serial numbers must be for the relevant version.
You can download the trial version of PSE9 from http://prodesigntools.com/photoshop-elements-9-premiere-elements-9-dir ect-download-links.html
Ensure that you follow the Very Important Instructions on that page before you click the download links otherwise the links will not work.
When you launch the PSE9 trial, enter your PSE9 serial number to license it permanently.