Elements 11 Organizer cannot work with more than one keyword at a time. Organizer 10 could. I don't understand why that feature was removed! It was extremely helpful to select multiple keywords to group pictures together. This needs to be put back!!
The way you can work with more than one keyword has been changed in the PSE11 new interface. You can select pictures by clicking on the > sign on the right of the keyword in the keyword panel, then select another keyword and do the same. That will show pictures having the first .AND. the second keyword.
The keyword panel at the top of the screen is another way to choose several keywords by checking checkboxes.
For more complex selections (not only .AND. but .OR. or other operators) the 'Find by detail menu' works like in previous versions.