When an Apple II starts up, it looks for a file named PRODOS. After it finds and loads PRODOS, it runs the first file on the disk that has the suffix .SYSTEM in its name. On the ProDOS Assembler Tools disk, the first such file is BASIC.SYSTEM, but the one needed is EDASM.SYSTEM.
Here's how to change the name of BASIC.SYSTEM so that EDASM.SYSTEM gets loaded first:
1. Make a copy of the ProDOS Assembler Tools disk.
2. Start up from the new disk.
3. When the Applesoft ] prompt appears, type the following, pressing Return after each line:
] UNLOCK BASIC.SYSTEM
] RENAME BASIC.SYSTEM,BASIC.SYS
4. Either restart the system, or type the following to start the assembler:
] -EDASM.SYSTEM
This tells BASIC.SYSTEM to load and run EDASM.SYSTEM. ProDOS Assembler Tools now starts up, as described in the manual.