I figured it out and it was easy. It just took a long time to figure that out.
All you need to do is add a new value into the config_options table. The category name appears to be where the item will show in the settings. I used general and it showed up under the general settings. Create a name for the variable, I used timeclock_url. Specify the type of interface, I used StringConfigHandler to get a textfield. The text for the field is specified in the language lang.php file.
I was adding a timeclock tab so in the lang.php file I put the following information.
'config option name timeclock_url'=>'Time Clock URL',
'config option desc timeclock_url'=>'Enter in the url for the timeclock.',
I created a file mysql_initial_data.php in the folder install/sql to add the config_options entry on the installation of the plugin. This file added the entry to the database.
Hope this helps.