I haven't actually ridden Greyhound, so I'm not exactly an expert on this, but I'll take a stab at it:
It appears the main terminal (open 24 hours, etc.) is 232 Forsyth Street SW, which is 3.6 miles from Earthlink. A cab ride of this distance should probably cost you in the neighborhood of $8.30 (not including tip).
Looking in the phone book, there is also a terminal listed at the Airport (though it's only open until 10 pm), and there are a few suburban locations listed (the Forsyth Street terminal is the closest to the venue). I'm assuming the ticket takes you first to the Forsyth Street terminal, and then they have shuttles to the satellite terminals (again, I don't have any idea how Greyhound works)?
Depending on what time your bus is due to arrive, if your ticket allows you the option of going directly to the Airport terminal first (BEFORE the bus proceeds to Forsyth Street), then you could take a Marta train from the Airport up to the Arts Center Station (N5). You wouldn't even have to change trains, and the cost would be about $1.75 one-way. This option is only valid under the assumption that your bus is not arriving during the middle of the night after the trains have stopped running (typically 12:30 am).
If the bus automatically goes to Forsyth Street first, then there is also one other possibility: according to the Greyhound website, the Amtrak station (1688 Peachtree St NW) serves as a "Limited Service Bus Stop". If your ticket can get you to this locale, this would be ideal, as the Amtrak station is about 0.5 miles from Earthlink, so you could walk it (or, if you needed to take a cab due to weather, etc. , then a cab would only cost you about $2.88 (not including tip)).
I'm not sure if any of this helps, but if you have additional questions, just post a follow-up, and I'll see if I can find out any additional information...