if the ownership to the community is yours, on the top of the lj home page, click "manage", "customize", and then there should be a wee drop down box at the top that says "work as user". choose the name of your community and away you go. that will allow to adjust the look of the community. to adjust the actual settings, it's simply "manage", "communities", and then "settings".

at least that's how it all works with my community. :)