Attention: this post refers to the CTP2 version of PerformancePoint Server 2007.
If, when opening Excel (2007 or 2003), after installing the PerformancePoint Add-In for Excel, you can't find the ribbon (2007) or the menu/task pane (2003) of PerformancePoint, then probably your Regional Settings are not set to English (United States). The CTP2 version of the Add-In for Excel only works with those regional settings, and if you configure your system to use different ones, the Add-In will not start.
Solution: Change the regional settings to English (United States).