The force.com excel connector relies on the Office Toolkit v3.0 COM, provided by Salesforce.com to communicate with the Force.com API via SOAP, therefore you must ensure that you have the Office Toolkit configured on your system.
Install the Salesforce Office Toolkit 3.0
Click on the following link to run the install (windows MSI installer):
Office Toolkit 3.0 to download and then run the installer.
After downloading, and un-ziping the sforce_connect.xla you must take care, the file itself is an Add-In and not intended to be opened directly by clicking on the XLA file, please resist the temptation. To Install the XLA file into Excel:
Save XLA file into a permanent folder, i use my documents, if you move this file later, you will defeat the install and the connector will no longer run
Launch Excel
Click on the Tools menu bar
Click on -> Add-Ins...
Click on the Browse button and locate the Add-In file from the directory where you unpacked the ZIP file,
select the add-in and click OK to load it.
When the Add-In is loaded you may see a message about Macro Security, you must have a security setting which will allow you to run Macros to access the functionality in the Sforce Connector.
When the Sforce Connector floating menu bar appears, the install is complete and you are ready to access the Sforce API