If this address is accessible by Marta?

This answer may not be as specific as you are looking for, but yes it is.

Take the North rail to the Sandy Springs station. From there it is a little over a 1/4 mile walk.

Here are directions from Yahoo Maps.

Start at 199 MOUNT VERNON CIR, SANDY SPRINGS going toward PEACHTREE-DUNWOODY RD NE – go 0.2 mi

Turn Right on PEACHTREE-DUNWOODY RD NE – go 0.1 mi

Arrive at 6600 PEACHTREE-DUNWOODY RD NE, ATLANTA, on the Left