AddIn.Installed Property (Excel)
True if the add-in is installed or to install the add-in, False if the add-in is uninstalled or to uninstall the add-in. Read/write Boolean.
A variable that represents an AddIn object.
Setting this property to True installs the add-in and calls its Auto_Add functions. Setting this property to False removes the add-in and calls its Auto_Remove functions.
This example uses a message box to display the installation status of the Solver add-in.
Set a = AddIns("Solver Add-In")
If a.Installed = True Then
MsgBox "The Solver add-in is installed"
MsgBox "The Solver add-in is not installed"