Here are the ways the Eclipse documentation states you can navigate the Content Assist list:
You can use the mouse or the keyboard (Up Arrow, Down A
If you try to fill in the content manually, Eclipse will move.
The search is quite advanced, e.g. "ArL" can match ArrayList, etc., so if you know what you are looking for, instead of navigation you could use search.
This is a great feature, and you can implement it like so: bind movement keys in Eclipse's key binder and then use them in the content assist menu!
Preferences/General/Keys
Line Down: Bind to Ctrl+K Line up: Bind to Ctrl+I
(both when 'Editing Java Source')
and then try moving with Ctrl-K/I when the content assist pops up!
In Kepler, as jed mentions in the comments, select "when editing text" instead and it should work.
This is certainly not a solution for everyone, but if you switch to the Emacs keyboard layout you can just use the key bindings for the "previous line" and "next line" commands, e.g., Ctrl-P and Ctrl-N.