Post Grid not displaying on my blog page

Hey guys,

I am having issues displaying post grid on my blog page. I have it set to all my preferred settings and I pasted the short code in my blog page. I can't figure out where to paste the PHP code. It says to paste it in the themes file, I have a customized theme that was created by a third party organization. I have watched multiple online videos and they all show a template drop-down box in the "page attributes group box" under the parent drop-down box, but I don't have the template drop-down.

Thanks!