The easiest way to open a PPTM file is to double-click on it and let your PC decide which default application should open the file. If no program opens the PPTM file then you probably don't have an application installed that can view and/or edit PPTM files.
Use Notepad or another text editor to open the PPTM file. Many files are text-only files meaning no matter the file extension, a text editor may be able to properly display the file's contents.