McTwist's addon enabler screwed me up when I used it.
Go into single player, enable, and all of a sudden, framerate would just drop like dubstep.
Instead of doing that, open your console up by pressing ~ on your keyboard(left-top corner), which opens the console.
Open your documents, then go into the blockland addons folder, look for the name of that addon you want.
In the console type
exec("add-ons/addonnamehere/server.cs");