From their main page, click on the RECORD SEARCH button and then the ARCHIVE SEARCH link. Here you will find a menu of items to begin your search. Many of the records will require you to locate your ancestor in the index, noting volume and/or page numbers to be entered back on the main page. Most of the indexes do not contain the alphabetical listings that we are used to using. You will find that surnames are listed under their first letter and then by the first letter of the first name. This requires you to read each name. Not doing so will cause you to possibly miss an important entry. Once you see how the indexes are set up, this is an easy task and with some patience you will find some information for your family tree.
I was very lucky in that I found a will of my ancestor, Nicholas Thill, which gave me the names of his before unknown children. I was also able to locate a couple marriage records on the site. I hope you have good luck as well.