This works more or less the same way in Windows 7 64-bit as it did in other versions. In Windows Explorer (i.e. the file browser) double-click on an m file. If the .m extension is not associated with another program (the way MS Access hijacks the .mat extension, for example) then a dialog box will open asking you to either use a web service to find the right program or for you to select it manually. Choose the latter, make sure the always use this program box is checked, and select the version of MATLAB you want to use.