this is what i would do........
first go edit your Nav template , look for this NAV ID "navbar" ( this is what im editing on vb.com with FB ) ..
before that place a new nav :
Code:
<div style="position: fixed; left: 50%; margin-left: -270px;">
just make sure you CLOSE THE DIV ! after the navbar DIV , this should work , but you need to do some more editing to make it look neat..