You can just drag and drop items directly into the list to add them. Click Login Options in the lower-left corner. If you arent logged in as an administrator, youre asked to enter an administrator name and password. Click the lock, then enter the account password.
It’s usually easier to just drag and drop though. Choose Apple menu > System Preferences, then click Users & Groups. To add a new application into the startup list, you can just drag and drop the icon into the list, or you can click the Plus icon and select an item using the file browser manually. I have the latest version of OS X 10.9.
Today, the same thing happens with OmniOutliner Pro (MAS Version). Removing items is as easy as selecting one and clicking the Minus button. Since a few days, after every boot Safari will automatically start. You can select an item in the list and click the Minus button to remove it from starting up, and you can check the Hide box to make the application not come to the foreground when it starts - it’ll basically start up, but minimize itself into the dock when it starts. Now you’ll see a list of items that will open automatically when you log in. Once there, find your user account on the left-hand side (although it will always be selected), and then click on the Login Items tab. Open up System Preferences, and then click on the Users & Groups icon.