The installation procedure for the Spreadsheet Add-In is a bit different - the installer is not the regular Oracle Universal Installer (OUI) that comes with Oracle products, nor does it use the OPatch route.
Once you click on the installable .exe file (OBISpAddinInst_10.1.2.1.47.exe), the first screen displayed is fairly simple - with two buttons, one to start the install, the other to cancel.
A default path is displayed, and you have the option of changing it.
A log file of the install process is written to "<install>\OracleBIAdd-InInstall.log" - which can be useful in the event of an unsuccessful installation.
If you have Excel running, this is the stage where errors will occur. One thing that I realized was that even if Excel does not seem to be running, it may still be alive as a process. So be sure to use your task manager to check for and end any Excel processes before you start the installation.
If you keep the 'View Supplemental Information' box checked, this page below is displayed upon clicking the 'Finish' button. The 'Quick Tour Demo' link launches a short Flash based demo of the Spreadsheet Add-In.
Should the installation fail for any reason, you should make sure that Excel is not running, and re-run the installation - if you choose the same directory to install the Add-In in, the existing files will be overwritten (which is no cause for worry).