Okay, it's a .rar so you need to unpackage it with either an app on your N7 like Unrar or something like WinRar on you PC (or equivalent if WinRar is not available on Mac).
To sideload an app you first need to check unknown sources...
Settings> Security> Device administration> tick Unknown sources.
Now the .apk file can be placed anywhere on the device, at the root of the internal user memory or in any folder including self created folder, you'll need a file manager on your Nexus 7 like Astro File Manager, ES Explorer or Solid Explorer. Simply locate the .apk where you placed it using your Mac and tap the .apk, the Android application installer should start and follow the steps to install the app.
I will add, be sure an application in .apk format came from a reliable source because downloaded .apk's from the web is one of the main sources of Android malware.
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