The obvious answer is to right-click on a file to open it.
If you mean open a file externally then I suggest you run acme from plan9port, then just run 'plumber' and use the program 'plumb' to send open commands to acme. It is fairly flexible:
$ plumb /path/to/file
$ plumb /path/to/file:35
$ plumb /path/to/file:/NameError
The nice thing is if the file is already open, acme will jump to the right place and highlight what you want.