Open XML Package Editor is one of the must tool for Office Developers and also my favorite tool when it comes to debugging Word/Excel related file composition or XML parts.
Current version which is available for download in Visual Studio gallery does't provide support for 2015. It supports only VS 2010/2012/2013.
After checking Q&A section, noticed Chris Rae suggested to get the code and compile for targeting VS2015. I just downloaded, complied and also validated the behavior - working fine. Thanks Chris for sharing your work !.
Uploaded the compiled version supporting 2015 here - http://1drv.ms/1HHXVBC {"OpenXMLEditor.vsix"}
Try it out and let me know if that helps you.