basically just walk in and say you're lookin' to work. i guess make sure they know you've got a bike and aren't there to do foot messenger stuff. in boston they'll make you register and get a courier liscense from police headquarters, i'm not sure how it works in philly.
your expectations are in the right place, and you'll probably get more dirty looks for starting now than if you had started in january, but stick with it and do the grunt work without *****ing and you'll be fine. i loved that job.
unless they've got a full stable of riders, if its anything like where i worked, they'll hand you an application and expect you to be ready to ride asap ; )
good luck!