SAP has confirmed this is an issue and will be fixed in BI4.2 SP04 Patch1. As a work around if you copy the .xlx file to Xcelsius install folder (C:\Program Files (x86)\SAP BusinessObjects\Xcelsius 4.0) and then add the addon from there it will work.
Bryan Baca
The BI 4.2 SP04 version of SAP Dashboards produces the following error when attempting to add InfoBurst dashboard connectors:
The Add-On you are trying to install is not valid.
This issue is under investigation.