LÖVE isn't as a library that you import into Lua. What it does is embed the Lua interpreter and expose its game functionality to that interpreter.
If you'd like to interactive play with the API, there's a couple of libraries built for debugging and live coding:
[EDIT] The debug function can also be useful in a limited context.
[EDIT EDIT] The love-console library was apparently built on an older version of LÖVE, but at least it's a starting point for building your own prompt.
If you're not specifically looking to work with Lua and a game-centric SDL-based framework, another fun one to try out is ruby-processing. It wraps JRuby, making it more convenient to access the Processing library, and has a mode where it will watch a sketch and recompile it every time you save the file. There's also a "live coding" mode but I haven't gotten it to work for me.
[EDIT x 3] Also look at PyProcessing. It has the advantage of having to not bundle JRuby, and can be imported into a standard python interpreter (provided you have all the dependency modules installed, which is Pyglet (which requires PyObjC on OS X).