The easiest way to install is using luarocks. Just type:
luarocks install whetlab
and you're done!
However, if you need to install from source:
First download the Lua Client either as a zip archive here or clone it using Git
git clone https://github.com/whetlab/Whetlab-Lua-Client
You will need to download the Lua dependencies for the client using luarocks. You can do this by typing
luarocks install luasec
luarocks install luasocket
luarocks install luajson
Now you just have to add whetlab to your LUA_PATH:
export LUA_PATH="$LUA_PATH;<path-to-the-client>/Whetlab-Lua-Client/?.lua"
Check out our Lua client API Reference for details on how to use the client or take a look at the examples subdirectory for example usage.