Your class declaration is wrong I think. Try changing the line below:
nav class="navbar navbar-static-top"
to
div class="navbar"
EDIT:
To centralise the nav elements, add "row" to the line below:
div class="container" => div class="container row"
Then set the ul tag class to also include span12, so
ul class="nav span12"
Now to set the position of your nav elements, set their classes to be span2, and offset the first one (probably use offset1) so you would have
li class="active span2 offset1"
li class="span2"
li class="span2"
and so on. Hope that works for you. You may have to fiddle around with the offsets/spans a bit but it should work fine. The row class divides the width of the container into 12 parts. You have 5 nav elements so if you set them each to span2 you should have enough to offset by 1 and get them centered.