We have just been made aware of an issue when trying to add a new add-on into SAP Dashboards (Xcelsius) version 4.2 SP4
Symptom: When you try and add the add-on through "Manage Addons" you see the error "Add-On Cannot Be Installed". Any addons you had installed prior to upgrading will continue to function as normal.
This has been reported to SAP on Note # 2475742 and a fix will be implemented in 4.2 SP4 Fix Pack 1.
It is possible to get around this issue whilst we are waiting for FP1. If you put the addon XLX into the root of the Xcelsius install folder (eg C:\Program Files (x86)\SAP BusinessObjects\Xcelsius 4.0) then when you go to Manage Addons you will be able to add the addon now.