Ken, last year there was an form online that you could print out and send to the league office. However, I went to the officials' page and the form is not there now. So I think your best bet is to go to
http://usbl.com/staff.cfm
and send an email to Lynn Casa by clicking on her "link". Tell her that you'd like to try-out and see if she will send you an application. I went through the application process last year. If you'd like to talk more about it, just email me. Good luck.
Chuck