SnapShot is built using the CryEngine 3 SDK and can be run in the GameSDK. To play, download and unzip the free SDK that can be obtained from here http://www.crydev.net/ (note that a prior account on CryDev is required, but it is completely free). Download SnapShot, unzip and place files in this file path "\Downloads\GameSDK\Levels". In the file path \Downloads\Bin64 there is an exe. called "GameSDK", double click on this and the cry game will load. Finally click on singleplayer, select the "SnapShot" file and click on "Load". Alternatively it can be run in the Editor to allow more exploration, the "Editor.exe" is located in the same file path as the "GameSDK". Once the editor has loaded open the "SnapShot" file.
To customise the HUD correctly go to the folder "\Downloads\GameSDK", select and unzip all files ending in .Pak. Then find the "Camera_Viewfinder.xml" in the SnapShot folder and move it to the folder UIElementstype in the file path "\Downloads\GameSDK\Libs\UI\UIElementstype". In the editor.exe type the following command into the console window r_DisplayInfo 0, then go to view, open view pane, flow graph. A window will pop up and in this window click on file and load, go to the SnapShot folder and load the "CustomHUD.xml" file then close the window. Then go to view, open view pane, UI Emulator and another window will pop up. Click on the "Reload all UIElement XMl files" button on the top left (looks like a circle made out of two arrows). Finally click on the "Elements" button on the right and double click on Camera_Viewfinder.xml and close the window.
Now click anywhere on the main/perspective window and press ctrl + g, to run the game.