Verified answer

Well you can't download Impress separately, you have to download the whole suite

http://download.openoffice.org/index.html

Did you save or export as a PowerPoint (.ppt) file? If it's still in the Open Office Impress native file, it won't work. Although Impress may be able to open PowerPoint files, the other way around is probably not going to work, so you have to save as or export (I'm not sure which Impress needs to do) as an MS PowerPoint file to open it in PowerPoint. Hope that helps.