Christiania is car free, and parking conditions outside can be hellish on good weather, so do what everyone else does and jump on a bike, if that doesn't cut it, take public transit; there are two options:
Take the metro to Christianshavn station, from there it's about a 6 minute walk 500m, when you exit the metro station continue straight ahead south until the 2nd intersection there is a furniture store on the corner turn left down east Prinsessegade and walk a couple of hundred meters, you won't miss it, if you have any sort of idea what you are looking for.
You can also bus opt for bus 66, which stops right outside the entrance, it leaves from both Vesterport and the central station, about every 10 minutes for most of the day.
