Routeplanner is a commandline programme and those don't have menu entries.
However, the routeplanner-gnome should have a menu item, but it may not be
where you expect to find it. It could even be in a submenu. It might be
under Accessories or even Other. I am in PCLinuxOS, so I can't check my own
menus at this time. It comes in two parts, an editor and routeplanner. I am
not sure if I remember how to use it. The interface is quite basic. You need
to open a database first and then add cities, etc. Then hit plan route, if
memory serves. There is likely some documentation online.