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:
The bus stop is about 3-4 blocks from the convention center.