Using WiX for installation development provides a simple way to quickly build installers, while maintaining the power to extend to the most difficult deployment scenarios. For the deployment requirements of an Excel Add-In you should read Deploying Application-Level Add-ins, on the MSDN site.

Firstly, it is required to register your COM Add-in. This can be done from the command line using the RegSvr32 executable. In WiX, all that is required is:

<File Id="Addin_Dll" Name="Addin.dll" Source="Addin.dll" KeyPath="yes" >
    <Class Id="{CLSID UUID}" Context="InprocServer32" Description="Addin" ThreadingModel="apartment" >
        <ProgId Id="Addin.Connect" Description="Connect Class" />
    </Class>
</File>

The Addin.dll is  a C++ COM component. Similarly can be done registering a .NET Assembly with COM Interop. The required registry keys are very simple with WiX to add:

<Registry Root="HKLM" Key="Software\Microsoft\Office\Excel\Addins\Addin.Connect" 
          Name="Description" Value="Description for Addin.Connect" Type="string" />
<Registry Root="HKLM" Key="Software\Microsoft\Office\Excel\Addins\Addin.Connect" 
          Name="FriendlyName" Value="Addin.Connect Friendly Name" Type="string" />
<Registry Root="HKLM" Key="Software\Microsoft\Office\Excel\Addins\Addin.Connect" 
          Name="LoadBehavior" Value="3" Type="integer" />

To register the add-in for just the current user on the computer, simply change the Root value to HKCU. It is also prudent to add a pre-installation condition that Excel in installed on the target machine:

<Property Id="P_EXCEL11INSTALLED">
    <RegistrySearch Id="SearchExcel11" Type="raw"
        Root="HKLM" Key="SOFTWARE\Microsoft\Office\11.0\Excel\InstallRoot" Name="Path" />
</Property>
<Condition Message="You must have Microsoft Office Excel 2003 installed to use this product.">
    P_EXCEL11INSTALLED
</Condition>

Adding these few blocks to your standard installation is all that is required in WiX to deploy an Office Excel Add-In.