There are lots of ground transportation options from the Austin Airport: http://www.ci.austin.tx.us/austinairport/transportation.htm

Cabs to the convention area run about $25 one way.
Shuttles to the convention area run about $13 one way.
Buses to the convention area run about $2 one way.

The bus schedule can be found here:
http://www.capmetro.org/riding/schedules.asp?f1=100
The bus stop is about 3-4 blocks from the convention center.