Loading addins when Excel is instantiated programmatically

Using CreateObject("Excel.Application") would have the same result as using New Excel.Application, unfortunately.

You will have to load the Addins that you need individually by file path & name using the Application.Addins.Add(string fileName) method.

I'm leaving this answer here for anyone else who ran into this problem, but using JavaScript.

A little background... In my company we have a 3rd party web app that used JavaScript to launch Excel and generate a spreadsheet on the fly. We also have an Excel add-in that overrides the behavior of the Save button. The add-in gives you the option of saving the file locally or in our online document management system.

After we upgraded to Windows 7 and Office 2010, we noticed a problem with our spreadsheet-generating web app. When JavaScript generated a spreadsheet in Excel, suddenly the Save button no longer worked. You would click save and nothing happened.

Using the other answers here I was able to construct a solution in JavaScript. Essentially we would create the Excel Application object in memory, then reload a specific add-in to get our save button behavior back. Here's a simplified version of our fix:

function GenerateSpreadsheet()
{
    var ExcelApp = getExcel();
    if (ExcelApp == null){ return; }

    reloadAddIn(ExcelApp);

    ExcelApp.WorkBooks.Add;
    ExcelApp.Visible = true;
    sheet = ExcelApp.ActiveSheet;

    var now = new Date();
    ExcelApp.Cells(1,1).value = 'This is an auto-generated spreadsheet, created using Javascript and ActiveX in Internet Explorer';

    ExcelApp.ActiveSheet.Columns("A:IV").EntireColumn.AutoFit; 
    ExcelApp.ActiveSheet.Rows("1:65536").EntireRow.AutoFit;
    ExcelApp.ActiveSheet.Range("A1").Select;

    ExcelApp = null;
}

function getExcel() {
   try {
       return new ActiveXObject("Excel.Application");
   } catch(e) {
       alert("Unable to open Excel. Please check your security settings.");
       return null;
   }
}

function reloadAddIn(ExcelApp) {
    // Fixes problem with save button not working in Excel,
    // by reloading the add-in responsible for the custom save button behavior
    try {
        ExcelApp.AddIns2.Item("AddInName").Installed = false;
        ExcelApp.AddIns2.Item("AddInName").Installed = true;
    } catch (e) { }
}

I looked into this problem again, and the Application.Addins collection seems to have all the addins listed in the Tools->Addins menu, with a boolean value stating whether or not an addin is installed. So what seems to work for me now is to loop through all addins and if .Installed = true then I set .Installed to False and back to True, and that seems to properly load my addins.

Function ReloadXLAddins(TheXLApp As Excel.Application) As Boolean

    Dim CurrAddin As Excel.AddIn

    For Each CurrAddin In TheXLApp.AddIns
        If CurrAddin.Installed Then
            CurrAddin.Installed = False
            CurrAddin.Installed = True
        End If
    Next CurrAddin

End Function