There are situations where other software will prevent the plugin installer from registering itself with Excel.
Here are the manual instructions:
First, it is necessary to know whether Excel is a 32-bit or 64-bit version. This can be found by clicking File | Account and selecting About Excel. On the first line of the About Microsoft Excel dialog, it should state whether it is 32-bit or 64-bit.
Next, click File | Options, and in the dialog that pops up select Add-Ins. At the bottom is a drop down labeled Manage:, and by default should be set to Excel Add-Ins. Click the Go... button next to the drop down.
Next, you should see the Add-Ins dialog, which has a checkbox list of the available add-ins. If somehow the incorrect Excel RFC plugin is installed, you should uncheck that if it is there. Then click Browse... and navigate to the folder C:\Program Files\Sawtooth Software\ExcelRFC. In that folder, there are two versions of the plugin specific to whether you have 32-bit or 64-bit Excel:
Please choose the one specific to the version of Excel. You can then click OK until you return to the main Excel window. The plugin should be working now, but you may need to reload the document or modify a level in the scenario to see the changes.