The "Advanced Search" option is no longer necessary if you are running DSpace with "Discovery" enabled. As of DSpace 4, the Discovery faceted/filtered search & browse is enabled by default. This is the main reason why we've removed that "Advanced Search" option from the user interface. This Discovery feature combines many of the aspects of the "advanced" and "simple" search into a single interface. You can easily type in simple searches, and then filter the search results (look for the "Advanced Filters" link in the results) using much more advance techniques.