Hatte exakt das gleiche Problem. Lösung ist:
ExcelApplication := CreateOleObject('Excel.Application');
i := ExcelApplication.AddIns.Count;
while i>0 do begin
if CompareText(ExcelApplication.AddIns[i].Name, 'MEIN_ADDIN.xll')=0 then
ExcelApplication.RegisterXLL(ExcelApplication.AddI ns[i].Fullname);
Dec(i);
end;