A c64 boots into basic. You can then enter and edit basic programs. In order to do ml, you need another program. There are many options, but the most used in the demo scene is turbo. This is an additional program that you run. This program allows you to enter and edit you ml before you execute it. Of course today most people use cross development and an emulator. Again there are several options, with cc65 probably one of the more popular ones. I wrote tmpx which allows you to use your favorite text editor and the turbo macro format, and then tmpx will compile it.