It seems Ive finally figured it out! Im going to outline this some below, to hopefully help anyone else who has problems with this.
2015 SOLIDWORKS API Help - SolidWorks.Interop.sldworks Namespace under DAssemblyDocEvents is where all your assembly events are listed.
I took the EventHandling.cs from the SWAddin template, added a method:
public int FileDrop(string filename)
Method stuff here!!!
Then added this line into the "override public bool AttachEventHandlers()" method in that same file.
doc.FileDropNotify += new DAssemblyDocEvents_FileDropNotifyEventHandler(FileDrop);
doc is an assembly doc variable, defined earlier in the class.
doc = (AssemblyDoc)document;
Bingo! It works.