This is likely an indication that OpenOffice 4 is not installed.Error creating slides for OpenOffice Impress. Object reference not set to an instance of an object.
If you intend to create slides for PowerPoint instead (e.g. using the Standard or Premium edition of PowerWorship 4.0) change the Presentation Software setting (on the Environment panel in the program settings) to Microsoft PowerPoint, instead of the default.
The default Presentation Software setting is OpenOffice Impress.
For OpenOffice installation instructions, refer to the PowerWorship 4.0 release notes: