With SharePoint 2010 & 2013, if you have access to your masterpage you can modify the number of dynamic navigation levels available. To do this, open the master page in SharePoint Designer and search for the ASPMenu tag. There could potentially be more than one in there, but the correct one will have an ID of "TopNavigationMenu" or something very close. The left (Quick Launch) menu will have an ID similar to "V5QuickLaunchMenu".
Once you find that, the tag will have an attribute called "MaximumDynamicDisplayLevels" which is set to 1 by default. You can change that to 2 or 3 as needed. Obviously, you don't want to go much higher, since multiple levels of menus turns into a mess fast.
There is also a setting called "StaticDisplayLevels" which affects how many levels of static links you can create.
Related Question: How to create multi level navigation?
UPDATE: Apparently if you have SharePoint 2013, there is a bug with structured navigation levels that was fixed with the August 2013 CU update.