Hi Sonia,
You need to apply a width to the menu container, then use margin:0 auto to center it. You are best off specifying the width in EM’s usually so that the menu expands with an increase in text size. Although this may not be a good idea if the menu may potentially extend beyond the width of it’s containing block.
Something like the following should work:
[code:1fbby93b]
#suckerfishnav {width:20em;margin:0 auto}
[/code:1fbby93b]