the multiplayer is free? ???
No you have to have a premium account.
There are some ways you can work around local multiplayer, but online multiplayer games are in no way free.
Multiplayer is not free, and it isn’t finished either. grazer is currently upgrading flowlab to a new engine and recoding it, so the multiplayer that is in the flash version of flowlab is just a test demo for premium users, it will be a while until a full version is built for HTML5.