Well, if Meigs was still around this would be pretty easy, but it isn't so choices are limited.
As per Scott's suggestion, fly into 3CK, get a lift to the Metra line in Crystal Lake (about 2 miles away), take the Metra NW-UP line to Irving Park Rd, and then the Wrigley Express bus. Cabs are also relatively easy to get at the Irving Park station. I've also walked it, but it is a hike, approx 4 miles.
You could also fly into MDW, get a lift around the airport to the Orange line EL train. Take that downtown, and then take the Red line to Wrigleyville. Warning, the Red Line trains can be really crowded.
http://www.transitchicago.com/assets/1/maps/ctatrainmap.png
A note about Chicago public transit. The metra trains are scheduled and run every hour or two (metrarail.com). In contrast the EL trains arrive often enough that a schedule isn't that important. Just wait for the next train.
Whatever you decide... Do NOT get a rental car and drive. It just isn't worth it. Traffic on I-90 is bad, and it is worse on gameday. The traffic gets even worse once onto the city streets near Wrigley. Then you'll get the pleasure of paying $20+ for parking, and may not be able to get your car out easily when the game is done (many lots stack the cars several deep).
Go Cubbies !