OK. You create a "Impress/Presentation" slide of your ER diagam. You go to File>Save. At the dialogue box, You uncheck "automatic file extension" and give the file a name with a .ppt extension. Now, at the Save as type box, there is a little down arrow which when clicked will have a drop down list of severl file types, incluing several versions of Powerpoint. Just choose the appropriate version. Note that you have to create the diagrams with the Open Office's Presentaion program, using the drawing tools therein. If you use the seperate "Draw" program(with similar drawing tools) to create the ER diagram, you will not be able to save it as a .ppt . One other thing to note is that yu also have to have MS Powerpoint installed.
(From a previous posting, it seems that you have created the ER diagram with the drawing program and then tried to save it as a Powerpoint Point file. Sorry, if I am wrong about that - but there still seems to be some confusion. I have just tried to do what I described - successfully)