Environmental Variables

Windows 7 - Control Panel, System, Advanced System Settings, Advanced Tab, Environment Variables.
These must be added to the User Variables and not the System Variables.

VSTO_EVENTLOGDISABLED - 0

You can use the event viewer to see messages captured by the VSTO runtime.
Applies to document level and application level projects in 2007 and 2010
0 - enables event logging (default)
1 - disables event logging
SS

VSTO_LOGALERTS - 1

This environment variable allows you to write the error messages to a log file.
For an add-in this log file (manifestname.manifest.log) is created in the folder that contains the deployment or application manifest
For a document-level solution this log file (documentname.extension.log, ExcelWorkbook1.xlsx.log) is created in the folder that contains the workbook or document.
This is only relevant if the add-in has full trust
1 - populate log
0 - suppress log (default)
If that fails then the log file is created in the local %TEMP% folder
SS

VSTO_SUPPRESSDISPLAYALERTS - 0

This environment variable allows you to display each error message in a message box.
This is only relevant if the add-in has full trust
0 - display alerts
1 - suppress alerts (default)
This displays the "Microsoft Office Application Add-in" dialog box when the application starts
SS

Event Viewer

Starting in Visual Studio 2008 SP1 error messages that are captured by the runtime will appear in the event viewer.
Control Panel > Administrative Tools > Event Viewer - Application
Event Source = VSTO 4.0 | VSTO 3.0
Event Source = .NET Runtime
SS

Sub CreateEnvironmentVariable() 
    Dim wshShell As Object 
    Dim wshUserEnv As Object 
    Set wshShell = CreateObject("WScript.Shell") 
    Set wshUserEnv = wshShell.Environment("USER") 

    ' Set the environment variable
    wshUserEnv("VSTO_SUPPRESSDISPLAYALERTS") = 0 
    MsgBox "Environment Variable VSTO_SUPPRESSDISPLAYALERTS was created", vbOKOnly + vbInformation, "Success" 

    Set wshUserEnv = Nothing 
    Set wshShell = Nothing 
End Sub 

Sub RemoveEnvironmentVariable() 
    Dim wshShell As Object 
    Dim wshUserEnv As Object 
    Set wshShell = CreateObject("WScript.Shell") 
    Set wshUserEnv = wshShell.Environment("USER") 

    On Error Resume Next 
    wshUserEnv.Remove ("VSTO_SUPPRESSDISPLAYALERTS") 
    If Err.Number = 0 Then 
        MsgBox "Environment Variable VSTO_SUPPRESSDISPLAYALERTS was removed", vbOKOnly + vbInformation, "Success" 
    Else 
        MsgBox "Environment Variable VSTO_SUPPRESSDISPLAYALERTS doesn't exist in the system", vbOKOnly + vbCritical, "Error"  
    End If 

    Set wshUserEnv = Nothing 
    Set wshShell = Nothing 
End Sub ()