How can I convert a catalogue from PSE on an old PC to a PSE 11 catalogue on another PC?

How did you transfer the pictures and catalog to the new PC ? The recommended way is to do a full backup on the old PC, and a restore on the new one.

http://helpx.adobe.com/photoshop-elements/kb/backup-restore-move-catalog-photoshop.html

The restore process should first restore the picture files, then convert the catalog to the new PSE11 format.

The problem is different if you install PSE11 on the same old computer, you don't have to move files, only convert to the old format catalog.