First, you don't open map files with sapien, you open scenario tags.
Default CE does not come with the single player tags. You can get these you can get them by downloading them from the site or rip them with HEK+. Ripping them can cause some issues with tags getting corrupted.
Download Tags: http://hce.halomaps.org/index.cfm?fid=6690
If you are going to change the scripts then you will need the zteam scripts: http://hce.halomaps.org/index.cfm?fid=3424
Open Sauce is not the only way to open single player scenarios. Unless your using the features OS gives you like shader extensions, script extensions, DLMs, boarding or actor transformations, you don't need OS. Normal sapien works just fine.
Also, the newest OS is here: https://bitbucket.org/KornnerStudios/opensauce-release/downloads