There's no way officially (or unofficially that I've heard of) to plug in external storage to the Apple TV - it is meant to be a network streaming box. If what you have really is a 2nd generation AppleTV, it's quite old - those shipped in September 2010 and were superseded by the 3rd gen in March 2012. The current 4th gen shipped in October 2015, a year and a half ago.
The 2nd and 3rd generations had a fixed set of apps/icons, provided by Apple, and you could only rearrange or hide them.
The 4th gen introduced a proper 3rd-party App Store, and you can download all sorts of apps. Including Infuse, Plex, and VLC, which are all designed to stream video (or audio) from a local network server, and do so quite well (Kodi is the major player that's missing - there are tutorials for how to shoehorn it onto the AppleTV, along with many complaints of trouble doing so, since it isn't coming in through the front door - there are better devices for running Kodi).
Streaming from a local network drive is the approach most people take these days (why move content to flash drives and plug them in when you get a multi-terabyte NAS for under $150, drop that on your network, and throw all your video there?). Note that you could also stream from your existing networked Mac/PC, as well, for no cost.
