Did you happen to notice this sticky?
And by visible signs we don't mean bubbling of the airlock, we mean krauzen or more importantly hydrometer readings.
Fermentation is not always "dynamic," just because you don't SEE anything happenning, doesn't mean that anything's wrong, and also doesn't mean that the yeast are still not working dilligantly away, doing what they've been doing for over 4,000 years....
If you've underpitched or not made a starter which it sounds like you didn't do, right now your yeast is cominng out of a dormant period, it finds it self surrounded by 5 gallons of food, so it is waking up, and waking up its friends...THEN before it starts truly diving in the yeast start growing an army to best eat it, so they have a wild orgy and then make a bunch of yeast babies. Then they get to work.
So that is why it can take up to three days before the really get going..it's called lag time, and it is perfectly normal.
An airlock is a vent for excess co2, nothing more, it's to keep your beer off the ceiling, and is designed to vent and still keep stuff out of your beer....that's really it...
In fact many no longer use airlocks at all, just cover the hole with a piece of tinfoil, or use a piece of plexiglass instead of a lid.
Read this for why airlock analysing is useless, and what is the only gauge of ferementation...http://www.homebrewtalk.com/1217925-post3.html
The only way to truly know what is going on in your fermenter is with your hydrometer.
Like I said here in my blog, which I encourage you to read, Think evaluation before action
you sure as HELL wouldn't want a doctor to start cutting on you unless he used the proper diagnostic instuments like x-rays first, right? You wouldn't want him to just take a look in your eyes briefly and say "I'm cutting into your chest first thing in the morning." You would want them to use the right diagnostic tools
before the slice and dice, right? You'd cry malpractice, I would hope, if they didn't say they were sending you for an MRI and other things before going in.....
Thinking about pitching more yeast before taking a hydrometer reading after 72 hours, is the same thing....thinking about a "cure" before we even know if the beer is even "sick."