Thankfully I solved this. If anyone else tries to use Fsm.EventData... in any C# script you must include:
using HutongGames.PlayMaker; //the namespace Fsm.EventData belongs to
at the top of your script. Otherwise Fsm.EventData is not recognized.
If you are making a custom action using Fsm.EventData (such as SetEventData.cs), then wrap your script in
namespace HutongGames.PlayMaker.Actions
{
}
This is now [SOLVED] if someone wants to update the Subject Title (or can I do that?).